Method for reducing NOx from exhaust gases produced by industrial processes

ABSTRACT

Gas-phase methods and systems for reducing NOx emissions and other contaminants in exhaust gases, and industrial processes using the same, are disclosed. In accordance with the present invention, hydrocarbon(s) autoignite and autothermally heat an exhaust gas from an industrial process so that NH 3 , HNCO or a combination thereof are effective for selectively reducing NOx autocatalytically. Preferably, the reduction of NOx is initiated/driven by the autoignition of hydrocarbon(s) in the exhaust gas. Within the temperature range of about 900-1600° F., the introduced hydrocarbon(s) autoignite spontaneously under fuel-lean conditions of about 2-18% O 2  in the exhaust gas. Once ignited, the reactions proceed autocatalytically, beating the exhaust gas autothermally. Under some conditions, a blue chemiluminescence may be visible.

This application is 371 of PCT/US97/19848, filed on Oct. 31, 1997 whichis a continuation of U.S. application Ser. No. 08/742,769, filed Nov. 1,1996 now U.S. Pat. No. 5,985,222.

FIELD OF THE INVENTION

The present invention relates to the removal of nitrogen oxides or “NOx”from exhaust gases and the like, and more particularly to processes andapparatus for reducing NOx selectively using autocatalytic, autothermalreactions in a manner to also remove other exhaust contaminants from thecombustion of carbonaceous fuels, and also to industrial processes usingthe same.

BACKGROUND OF THE INVENTION

Without being bound by any particular theory, the background of thepresent invention will be described by way of a description ofparticular problems discussed in the art and various proposed solutionsto such problems. For brevity, various references will be briefly andgenerally summarized herein. A more complete understanding of suchbackground art may be obtained by a complete review of the documentscited herein, etc. What should be understood from the followingdiscussion is that, despite such extensive prior efforts to providevarious methods of NOx removal and the like, a continuing need existsfor practical and low-cost methods of NOx removal in a variety ofindustrial processes, which may utilize a variety ofcommercially-available reductants.

Carbonaceous fuels are burned in internal combustion engines and otherequipment such as boilers, furnaces, heaters and incinerators, and thelike (i.e., in a wide variety of industrial process). Excess airfrequently is used to complete the oxidation of combustion byproductssuch as carbon monoxide (CO), hydrocarbons and soot. High temperaturecombustion using excess air, however, tends to generate oxides ofnitrogen (often referred to as NOx).

Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO₂).Free radicals of nitrogen (N₂) and oxygen (O₂) combine chemicallyprimarily to form NO at high combustion temperatures. This thermal NOxtends to form even when nitrogen is removed from the fuel. Combustionmodifications which decrease the formation of thermal NOx generally arelimited by the generation of objectionable byproducts.

Mobile and stationary combustion equipment are concentrated sources ofNOx emissions. When discharged to the air, emissions of NO oxidize toform NO₂, which tends to accumulate excessively in many urbanatmospheres. In sunlight, the NO₂ reacts with volatile organic compoundsto form groundlevel ozone, eye irritants and photochemical smog. Theseadverse effects have prompted extensive efforts for controlling NOxemissions to low levels. Despite advancements in fuel and combustiontechnology, groundlevel ozone concentrations still exceed federalguidelines in many urban regions. Under the Clean Air Act and itsamendments, these ozone nonattainment areas must implement stringent NOxemissions regulations. Such regulations will require low NOx emissionslevels that are attained only by exhaust aftertreatment.

Exhaust aftertreatment techniques tend to reduce NOx using variouschemical or catalytic methods. Such methods are known in the art andinvolve nonselective catalytic reduction (NSCR), selective catalyticreduction (SCR) or selective noncatalytic reduction (SNCR).Alternatively, NO may be oxidized to NO₂ for removal by wet scrubbers.Such aftertreatment methods typically require some type of reactant forremoval of NOx emissions.

Wet scrubbing of NO₂ produces waste solutions that represent potentialsources of water pollution. Wet scrubbers primarily are used for NOxemissions from nitric acid plants or for concurrent removal of NO₂ withsulfur dioxide (SO₂). High costs and complexity generally limit scrubbertechnology to such special applications. Wet scrubbers are applied tocombustion exhaust by converting NO to NO₂, such as is described in U.S.Pat. No. 5,047,219.

The NSCR method typically uses unburned hydrocarbons and CO to reduceNOx emissions in the absence of O₂. Fuel/air ratios must be controlledcarefully to ensure low excess O₂. Both reduction and oxidationcatalysts are needed to remove emissions of CO and hydrocarbons whilealso reducing NOx. The cost of removing excess O₂ precludes practicalapplications of NSCR methods to many O₂-containing exhaust gases.

Combustion exhaust containing excess O₂ generally requires chemicalreductant(s) for NOx removal. Commercial SCR systems primarily useammonia (NH₃) as the reductant. Chemical reactions on a solid catalystsurface convert NOx to N₂. These solid catalysts are selective for NOxremoval and do not reduce emissions of CO and unburned hydrocarbons.Excess NH₃ needed to achieve low NOx levels tends to result in NH₃breakthrough as a byproduct emission.

Large catalyst volumes are normally needed to maintain low levels of NOxand NH₃ breakthrough. The catalyst activity depends on temperature anddeclines with use. Normal variations in catalyst activity areaccommodated only by enlarging the volume of catalyst or limiting therange of combustion operation. Catalysts may require replacementprematurely due to sintering or poisoning when exposed to high levels oftemperature or exhaust contaminants. Even under normal operatingconditions, the SCR method requires a uniform distribution of NH₃relative to NOx in the exhaust gas. NOx emissions, however, arefrequently distributed nonuniformly, so low levels of both NOx and NH₃breakthrough may be achieved only by controlling the distribution ofinjected NH₃ or mixing the exhaust to a uniform NOx level.

NH₃ breakthrough is alternatively limited by decomposing excess NH₃ onthe surface of a catalyst as described in U.S. Pat. No. 4,302,431. Inthis case, the excess NH₃ is decomposed catalytically following aninitially equivalent decomposition of NOx and NH₃ together. Thedecomposition of excess NH₃, however, reduces the selectivity of the SCRmethod, increasing the molar ratio of NH₃ with respect to NOx as much as1.5 or higher.

In a combination of catalytic and noncatalytic reduction methods, bothNOx and NH₃ removal may be controlled by SCR following an initial stageof NOx reduction by SNCR. In the SNCR method, NOx emissions may bereduced partially without controlling NH₃ breakthrough to a low level.The SCR method may decrease NOx further while also lowering NH₃breakthrough to an acceptable level.

The use of excess NH₃ to enhance NOx removal by the SNCR method isdescribed in detail in U.S. Pat. Nos. 4,978,514 and 5,139,754. With suchmethods, the NH₃ injection to SNCR is controlled so that the unreactedNH₃ remains sufficient for the subsequent catalytic reduction of NOx toa low level. This injection strategy is based on the use of excess NH₃for reducing NOx to lower levels, as with the SCR method describedabove.

Another method for combining SNCR and SCR methods is described in U.S.Pat. No. 5,510,092. In this method, the catalytic NOx reduction isalways maximized using a separate NH₃ injection grid, and the NOxemissions are reduced noncatalytically only as needed to maintain afinal low NOx level. This method decreases the consumption of NH3 byminimizing the use of SNCR which removes NOx less selectively than thecatalytic method.

The low selectivity of the SNCR method and the use of excess NH₃ fordecreasing NOx levels is reported by Lyon, who is believed to have firstsuggested the noncatalytic reduction of NOx (U.S. Pat. No. 3,900,554).In commercial coal-fired boiler tests, 73% NO reduction has beenreported with 2.2 ppm NH₃ breakthrough using a 0.9 molar ratio of NH₃with respect to NO, while 86% NO reduction required 11 ppm NH₃breakthrough and a 2.2 molar ratio of NH₃ with respect to NOx. Theseresults are reported in Environ. Sci. Technol., Vol. 21, No. 3, 1987.

In another article (Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986), Lyonalso reports the inhibiting effect of NH₃ on CO oxidation. Thisobservation in experiments and commercial tests is confirmed by modelingstudies. The inhibition has been explained in terms of competitionbetween NH₃ and CO for reaction with the OH free radical. It is believedthat, while NH₃ inhibits the oxidation of CO, the CO also decreases theselectivity of NO reduction by NH₃.

This “sacrifice of residual CO oxidation” is described by Lyon as animportant limitation of the noncatalytic reduction method. According tothese teachings, the injection of NH₃ should follow the completion of COoxidation in order to overcome this limitation. When NH₃ is injectedbefore the completion of CO oxidation, the oxidation of residual COtends to diminish, which may result in greater levels of byproduct COemissions.

Despite this disadvantage of greater byproduct CO emissions, manypatents teach the use of CO or other ancillary reducing materials tolower the effective temperature for reducing NO noncatalytically. Forexample, the use of CO to lower the temperature for NO reduction by HNCOis discussed in U.S. Pat. No. 4,886,650 as follows: “Where it is desiredto lower the operating temperature to a greater degree, larger amountsof CO or other H atom generating species will be added or vice-versa.”

This lowering of the effective temperature for NO reduction has beenrepeated generally in a consistent manner throughout patent literaturerelated to the noncatalytic method. The original discoveries of NH₃(U.S. Pat. No. 3,900,554) and urea (U.S. Pat. No. 4,208,386) as NOxreductants in the temperature range of 1600 to 2000° F. both reportedthe use of ancillary reducing materials to enable noncatalytic NOreduction throughout the temperature range of 1300 to 2000° F. Hydrogen(H₂), CO, and hydrocarbon(s), including oxygenated hydrocarbons, havebeen proposed as ancillary reducing materials that may lower theeffective temperature for noncatalytic NO reduction by NH₃ or urea. Thisuse of hydrocarbon(s) and CO is reportedly limited, however, due toincomplete oxidation, resulting in the production of air pollutants.Hydrogen has been cited as the preferred reducing material because itdoes not produce any air pollutants.

The use of hydrogen is limited because it decreases the selectivity forNO reduction by NH₃ or urea. To overcome this limitation, the hydrogenmay be added in successive multiple stages as described in U.S. Pat. No.3,900,554. A more detailed description of a multi-stage method fornoncatalytic NO reduction using NH₃ and hydrogen is disclosed in U.S.Pat. No. 4,115,515. This multi-stage method typically requires two ormore locations along the flowpath of the exhaust gas to inject reducinggas mixtures. The optimum use of multiple injection stages andalternative reducing gas mixtures depends on the exhaust gas temperaturein the vicinity of each injection location. The multi-stage methodaccounts for temperature gradients along the gas flowpath as well asvariations in temperature at each injection location.

Generally according to the these teachings, NH₃ is injected as the onlyreducing gas at temperatures in the range of 900 to 1000° C. (about 1650to 1850° F.), while mixtures of NH₃ and hydrogen are injected attemperatures in the range of 700 to 900° C. (about 1300 to 1650° F).Decreasing the NO concentration in the first injection stage using NH₃alone minimizes the less-selective reduction of NO in the second stagewhere hydrogen is used as the ancillary reducing material.

Patents have proliferated since such disclosures of noncatalyticreduction methods. In particular, U.S. Pat. Nos. 4,731,231, 4,800,068,4,886,650 and 4,908,193 have disclosed the decomposition of cyanuricacid, (HNCO)₃, to generate isocyanic acid (HNCO) for NO reduction. Also,other patents (for example, U.S. Pat. Nos. 4,719,092, 4,751,065,4,770,863, 4,803,059, 4,844,878, 4,863,705, 4,873,066, 4,877,591,4,888,165,4,927,612 and 4,997,631) have disclosed a variety of reducingmaterials as alternatives to NH₃, urea or cyanuric acid, or as enhancersfor use with NH₃, urea or cyanuric acid.

Such patents primarily address the acute problems of NH₃ breakthroughand byproduct CO emissions that are characteristic of the noncatalyticreduction method. In addition to the disclosures of various reductantsand enhancers, other patents (for example, U.S. Pat. Nos. 4,777,024,4,780,289, 4,863,704, 4,877,590, 4,902,488, 4,985,218, 5,017,347 and5,057,293) describe elaborate control strategies and multi-stageinjection methods.

Such control strategies and multi-stage methods primarily addressvariations in temperature. Combustion equipment typically operatethroughout a load range, and exhaust gas temperatures generally increaseat higher loads. Therefore, local temperatures vary at the fixedlocations where the reductant(s) and reducing material(s) are injectedinto the exhaust gas. The noncatalytic methods do not control the localtemperature for NOx reduction.

With noncatalytic reduction methods, the local temperature typically isused as a means for controlling the injection of reductant(s) andancillary reducing material(s). The patents teach the use of ancillaryreducing material(s) to lower the effective temperature for NOxreduction so that it matches the actual local temperature, which dependssolely on the production of the exhaust gas. It is important to notethat, in such teachings, the ancillary reducing material(s) are notinjected to control the local temperature. Ancillary reducingmaterial(s) may enable NOx reduction at lower effective temperatures,but may result in the formation of objectionable byproducts. Suchteachings tend only to minimize disadvantages of noncatalytic reductionmethods. Such techniques in general do not provide for concurrentdepletion of NH₃ and CO emissions.

The use of oxygenated hydrocarbons is described in U.S. Pat. No.4,830,839 as a means for scrubbing NH₃ breakthrough from a previousstage of noncatalytic NOx reduction. With this method, vaporizedconcentrations of oxygenated hydrocarbons in the range of 2 to 500 ppmare added to the exhaust gas so that their weight ratios with respect toNH₃ are in the range of 2 to 200. U.S. Pat. No. 5,047,219, however,subsequently discloses that oxygenated hydrocarbons oxidize NO to NO₂ attemperatures below about 1600° F.

Lowering the effective temperature for noncatalytic reduction belowabout 1700° F. also slows the thermal decomposition of nitrous oxide(N₂O) as described in U.S. Pat. No. 5,048,432. This patent teachesreheating of exhaust gases using a burner with a separate source ofcombustion air. The N₂O is decomposed thermally when the hightemperature flue gases from the burner mix and reheat the primaryexhaust gas above 1700° F.

Formation of N₂O as a noncatalytic reduction byproduct is described inU.S. Pat. No. 4,997,631. When NOx emissions are reduced by thenoncatalytic method, a portion of the reduced NOx is converted to N₂O.As described above, N₂O levels decrease at higher temperatures, but alsothe reported data suggest much less N₂O formation when NH₃ is used asthe chemical reductant. Urea and cyanuric acid reportedly result inhigher N₂O levels.

A different method for staging noncatalytic NO reduction is described inU.S. Pat. No. 3,867,507. Hydrocarbons are disclosed to reduce NO whenthe molar ratio of O₂ with respect to carbon is less than 2.5. Such lowO₂ levels, however, tend to result in the formation of objectionablebyproducts, including NH₃, hydrogen cyanide (HCN), CO and unburnedhydrocarbons. Such byproducts are removed by oxidation using added airat elevated temperatures, e.g.,1100° C. (about 2000° F.), in a secondstage.

Similar methods for staging noncatalytic NOx reduction are disclosed inU.S. Pat. Nos. 4,851,201 and 4,861,567. With these methods, reductant(s)are mixed with the exhaust gas and decomposed under fuel-rich combustionconditions in a first stage, and then NOx is reduced in a second stagewith an excess of oxygen. The temperature and oxygen concentration areadjusted between the two stages. The temperature ranges for each stagedepend on whether the reductant is cyanuric acid rather than NH₃ orurea.

Another method for lowering the effective temperature for noncatalyticNOx reduction is described in Int. App. No. PCT/US92/07212 (Pub. No. WO93/03998). It is suggested that hydrocarbons be injected so as to createstratified mixtures effective for generating partial oxidation productsas ancillary reducing materials to lower the effective temperature forNOx reduction using cyanuric acid.

Based on such previous teachings, noncatalytic NOx reduction byhydrocarbon(s) alone tends to be limited to fuel-rich combustionconditions, i.e., low O₂ levels. In the presence of excess O₂,hydrocarbon(s) and CO are ineffective for NOx reduction, but may be usedto lower the effective temperature for noncatalytic NOx reduction usingchemical reductant(s). In such an ancillary role, such materials alsoare claimed to lower NH₃ breakthrough, but such would be achieved onlyat the expense of decreased selectivity for NOx reduction.

Under these conditions, noncatalytic NOx reduction tends to be limitedbecause excess NH₃ breakthrough is needed to achieve low NOx levels, butthe oxidation of CO is inhibited by NH₃, so the lowering of noncatalyticreduction temperature using ancillary materials tends to increasebyproduct CO emissions. Both NH₃ and CO are objectionable SNCRbyproducts. In general, even the most elaborate proposed SNCR controlsand staging methods cannot deplete these objectionable byproductsconcurrently. In general, such methods only minimize the disadvantageousproduction of one byproduct at the expense of increasing another.

Furthermore, noncatalytic reduction methods are highly dependent upontemperature, but tend to provide no means for controlling this keycondition. Staging methods and elaborate controls are needed to maintainan effective temperature for the chemical reductant(s) when excess O₂ ispresent at the local exhaust gas conditions. Injecting reductant(s)under specific fuel-rich combustion conditions also is claimed torequire a staged introduction of excess air to complete the combustionof primary fuel.

Despite such staging methods and elaborate controls, the NH₃breakthrough from SNCR in general is depleted only by using a subsequentcatalytic method. Such SCR methods, however, do not remove theobjectionable byproduct CO emissions from SNCR without using a separateoxidation catalyst similar to NSCR. Alternatively, this disadvantage ofSNCR may be minimized to the greatest extent by limiting noncatalyticreduction only to maintain a final low NOx level in combination with SCRas described in U.S. Pat. No. 5,510,092.

Relegating the inherent advantages of a gas-phase method for reducingNOx to a subordinate role, however, does not minimize the keydisadvantages of using solid catalysts. The expensive installation oflarge catalyst volumes intrudes adversely upon the combustion equipment.The catalyst bed adds pressure drop, and the vaporization of NH₃ mayderate the combustion equipment by as much as 2%.

As is well known, solid catalysts tend to gradually become plugged andpoisoned under normal use and require periodic replacement. Prematurereplacement is needed when the catalyst is sintered or poisoned due tounusually high temperatures or contaminant levels resulting fromcombustion-related problems or other equipment failures. Like SNCRmethods, catalytic reduction methods are highly dependent upon ontemperature, but provide no means for controlling this key condition.

In considering the foregoing teachings and efforts, and in particular inview of the continuing need in numerous applications for practical,cost-effective reduction of NOx despite the extensive previous efforts,a need exists for new methods for selective NOx reduction, which may ingeneral combine certain advantages of previously disclosed methodssubstantially, but without disadvantages thereof.

As will be described hereinafter, Applicants submit that they havediscovered such methods.

It should be understood that the foregoing discussion of the backgroundof the present invention, and the detailed description of the presentinvention to follow, is provided for understanding the context andapplicability of the present invention, and is provided without beingbound by any particular theory or the like. References to particularbackground patents or other materials are for general discussionpurposes only and based on Applicants' understanding thereof, and thecomplete references should be consulted for the actual contents of suchpatents and materials.

SUMMARY OF THE INVENTION

The present invention provides new gas-phase methods for reducing NOxemissions and other contaminants in exhaust gases, and for industrialprocesses using the same. With methods in accordance with the presentinvention, hydrocarbon(s) autoignite and autothermally heat an exhaustgas so that NH₃, HNCO or a combination thereof are effective forselectively reducing NOx autocatalytically. These new autocatalyticmethods are distinguished by the self-sustained conversion of reactantswhen at least one reaction product acts as a catalyst so that thereactions proceed faster with formation of the catalyst and continueuntil reactants are depleted substantially.

With methods in accordance with preferred embodiments of the presentinvention, the reduction of NOx is driven by autothermally heating theexhaust gas to generate the effective catalytic species forself-sustaining the reactions until reactants are depletedsubstantially. Within the temperature range of about 900-1600° F.,hydrocarbon(s) are introduced to autoignite under generally uniformfuel-lean conditions with about 2-18% O₂ in the exhaust gas. Onceignited, the reactions proceed autocatalytically, heating the exhaustgas autothermally. Under some conditions, a blue chemiluminescence maybe visible.

Such single-stage, autocatalytic methods in accordance with the presentinvention need not depend on the order in which the reductant(s) andhydrocarbon(s) are introduced into the exhaust gas. Contrary to previousteachings, autocatalytic methods in accordance with the presentinvention do not require fuel-rich combustion or multiple reactionstages. The NH₃, HNCO or a combination thereof may be introduced orgenerated from reductant(s) before or during the fuel-lean autothermalconversion of hydrocarbon(s) and CO in the exhaust gas.

Autocatalytic methods in accordance with the present invention reduceNOx and deplete both CO and NH₃ in a substantially concurrent manner.These autocatalytic reactions are self-sustained by the autothermalheating of the exhaust gas following the substantially uniformautoignition of the hydrocarbon(s). Gas-phase methods in accordance withthe present invention may be advantageously applied without a solidcatalytic surface. Self-sustaining autothermal reactions in the gasphase may serve to partially remove other exhaust gas contaminants,including hydrocarbons, particulate matter and CO.

Methods in accordance with the present invention may be considered tocombine advantages of known methods for reducing NOx selectively, but,unexpectedly, without the disadvantages of solid catalytic surfaces,hazardous wastes or byproduct emissions, etc. Contrary to previousteachings, autocatalytic methods in accordance with the presentinvention interchangeably may use reductant(s) that consist of ordecompose to generate NH₃, HNCO or a combination thereof. In addition,hydrocarbon(s) that may be used in embodiments of the present inventionmay consist of the same liquid, gaseous or vaporous fuels that arecombusted to produce the exhaust gas containing NOx in the industrialprocess.

Also contrary to previous teachings, hydrocarbons and CO do not serve tolower the effective temperature range for reducing NOx by theautocatalytic method. With autocatalytic methods in accordance with thepresent invention, the exhaust gas is heated autothermally by both thepartial oxidation of hydrocarbon(s) to generate CO and the oxidation ofCO to CO₂. The introduction of NH₃ HNCO or a combination thereof duringthis autothermal heating results in NOx reduction, and thehydrocarbon(s), CO and NH₃ are depleted together in the same temperaturerange of about 1400-1550° F. Within this range, the depletion ofhydrocarbon(s), CO and NH₃ depends primarily on the final temperaturefor autothermal heating of the exhaust gas.

Also contrary to previous teachings, the autocatalytic method is notlimited by the inhibition of CO oxidation. Autocatalytic reactions maybe self-sustained while CO and NH₃ are depleted together whenhydrocarbon(s) autoignite and heat the exhaust gas autothermally to thetemperature range of about 1400-1550° F. In accordance with the presentinvention, NH₃ may be depleted below even 2 ppm concurrently with COremoval below about 50 ppm.

Also contrary to previous teachings, NOx emissions are reduced to lowlevels while the NH₃ is depleted substantially. In accordance withautocatalytic methods of the present invention, NOx emissions may bereduced about 80-90% to about 50-200 ppm using NH₃ and HNCO nearlystoichiometrically. Furthermore, in preferred embodiments NOx emissionsmay be reduced by as much as 99% to levels as low as about 10 ppm usingno more than about twice the stoichiometric ratio of NH₃ and HNCOrelative to NOx.

Such uniquely concurrent gas-phase removal of NOx, NH₃, HNCO,hydrocarbon(s) and CO in general is not highly dependent on the chemicalreductant(s). Similar results have been obtained in accordance with thepresent invention using NH₃, cyanuric acid, urea or decompositionproducts of urea. While the conversion of NOx to N₂O may depend on thechemical reductant(s), if desired byproduct N₂O emissions may be reducedto low levels using NH₃ rather than other chemical reductant(s).

In preferred embodiments of the present invention, the introduction ofhydrocarbon(s) is controlled to maintain a final reaction temperature inthe range of about 1400-1550° F. The autothermal heat release increasesthe exhaust gas temperature adiabatically in the absence of heat losses,or alternatively heat transfer surfaces may recover heat from theexhaust gas during the autothermal heating. Such heat recovery, however,should not cool the exhaust gas so excessively as to extinguish theautothermal reactions.

An autothermal heat release equivalent to an adiabatic temperatureincrease in the range of about 50-500° F. is preferentially utilized inpreferred embodiments to achieve a final exhaust gas temperature in therange of about 1400-1550° F. for implementing autocatalytic methods inaccordance with the present invention. The amount of hydrocarbon(s)introduced depends primarily on the initial exhaust gas temperature andany recovery (recycling) of heat released by the autothermal reactions.

Autocatalytic methods in accordance with the present invention typicallyutilize residence times no longer than about 1.5 seconds when theinitial exhaust gas temperatures are in the range of about 900-1600° F.As more fully describe elsewhere herein, CO and NH₃ typically aredepleted faster when the autothermal heating is initiated at highertemperatures in the range of 1050-1600° F. In this case, reactionresidence times in the range of about 0.02-1.0 seconds typically may besufficient to deplete both CO and NH₃ substantially. In accordance withthe present invention, higher initial exhaust gas temperatures in therange of about 1200-1600° F. enable substantial CO and NH₃ depletionwithin the range of about 0.02-0.5 seconds.

In accordance with the present invention, the introduction ofhydrocarbon(s) decreases beneficially when the exhaust gas is preheatedto the temperature ranges of about 1050-1600° F. or about 1200-1600° F.In these cases, the autothermal heat release need not exceed an amountequivalent to an adiabatic increase of about 50-350° F. or about 50-200°F., respectively, so long as the exhaust gas is heated autothermally toa final temperature in the range of about 1400-1550° F. In accordancewith the present invention, in certain embodiments this preheating ofthe exhaust gas also may improve the selectivity of NOx reduction.

With the present invention, the initial exhaust gas temperatures do notdepend on how the exhaust gas is preheated or cooled, so long as the O₂concentration is maintained in the range of about 2-18% by volume. Theexhaust gas may be heated or cooled initially using heat transfersurfaces, including any of various methods for preheating the exhaustgas by recovering heat after the exhaust gas is treated usingautocatalytic methods as provided herein. In alternative embodiments,the exhaust gas is heated directly by the combustion of a supplementalfuel in the exhaust gas.

In such alternative embodiments, the combustion of a supplemental fuelusing excess air also may enrich the O₂ concentration in an otherwise0₂-deficient exhaust gas. In this case, the supplemental fuel combustionmay serve the dual purpose of preheating the exhaust gas and enrichingits O₂ concentration. The combustion of a supplemental fuel also mayserve to preheat a portion of the exhaust gas to ignite moresupplemental fuel which is combusted directly in the exhaust gas. If theexhaust gas is preheated using fuel-rich combustion, autocatalyticmethods in accordance with the present invention may serve to remove inpart or substantial whole the additional contaminants from the fuel-richcombustion. In this context, it is important to note that the“supplemental fuel” is combusted for the purpose of preheating theexhaust gas, and not for chemically enhancing the NOx reduction.

Autocatalytic methods in accordance with the present invention may beused in combination with various modifications to the combustion processwhich generates the exhaust gas. In certain embodiments, suchmodifications may advantageously lower NOx emissions to decrease theintroduction of reductant(s) in accordance with the autocatalyticmethods of the present invention. In certain embodiments, combustionmodifications may beneficially maintain exhaust gas temperatures withinthe range of about 900-1600° F., or preferably about 1200-1600° F. forimplementing autocatalytic methods of the present invention. In certainembodiments, combustion modifications may maintain the O₂ concentrationabove about 2% by volume for implementing autocatalytic methods of thepresent invention.

Autocatalytic methods in accordance with the present invention also maybe implemented in conjunction with the primary combustion process sothat the autothermal heat release is recovered beneficially. Inalternative embodiments, for example, existing or new surfaces in a heatexchange boiler may serve to recover the autothermal heat releasegenerated in accordance with autocatalytic methods as provided herein.In such embodiments, the autothermal heating may replace primary fuelfor the purpose of generating steam or cracking petrochemicals (asexemplary industrial applications), or the autothermal heating may serveto increase the generating capacity of an existing boiler, etc.

In accordance with still other embodiments, combustion modificationssuch as over-fire air are utilized to enable lower NOx emissions fromthe primary fuel while also enriching O₂ in the exhaust gas. In the caseof coal-fired boilers, the replacement of primary fuel by autothermaloxidation may serve to increase furnace O₂ levels beneficially for thepurpose of decreasing unburned carbon on fly ash. Such benefits ofalternative embodiments of autocatalytic methods in accordance with thepresent invention may serve to increase overall boiler efficiency whilealso enhancing the value of byproduct fly ash, possibly avoiding thegeneration of an otherwise solid waste.

Autocatalytic methods in accordance with the present invention also maybeneficially consume NH₃ breakthrough from a previous exhaust gastreatment using SNCR, for example. In such embodiments, autocatalyticmethods in accordance with the present invention may serve to replacethe use of SCR as a means for controlling NH₃ breakthrough from SNCR.Such uses of embodiments of the present invention, however, also maypreferably apply autocatalytic methods as provided herein in place ofSNCR in order to reduce NOx more selectively. It is submitted that, asone exemplary advantage, the better selectivity of autocatalytic methodsas provided herein may greatly decrease the introduction and cost ofreductant(s), while substantially depleting both CO and NH₃, andreducing NOx emissions to low levels.

Since autocatalytic methods in accordance with the present invention mayreduce NOx emissions below most regulatory requirements, application ofthe present invention may replace the need for expensive catalystsaltogether. Such autocatalytic activity for removing NH₃ and CO alongwith NOx may be self-sustained and conducted in a manner so as to notdeteriorate with use like solid catalysts. As a result, autocatalyticmethods in accordance with the present invention may avoid the need toreplace existing catalysts poisoned by exhaust contaminants.

If emissions regulations require additional NOx reductions, thenautocatalytic methods in accordance with the present invention may serveto enhance SCR applications while minimizing catalyst volume. Inaddition to reducing NOx before SCR, autocatalytic methods as providedherein also may decrease contaminants such as hydrocarbons and sootwhich may foul catalytic surfaces. In such embodiments utilizing acombination of treatments, autocatalytic methods as provided herein mayenable the use of more efficient or cost-effective catalyst beds due toboth contaminant removal and the control of exhaust gas temperatures.

In a preferred combination of autocatalytic and catalytic reductionmethods, autothermal heating may continuously decrease hydrocarbon andsoot contaminants while controlling the exhaust gas temperature to thecatalyst. Emissions of NOx may be maintained at a desired level using aseparate injection of NH₃ ahead of the catalyst to minimize reductant(s)introduced to the autocatalytic method. Since autocatalytic NOxreduction need not exceed about 80-90%, reductant(s) may be consumednearly stoichiometrically, and CO emissions may be substantiallydepleted in the shortest time possible.

Accordingly, it is an object of the present invention to addressproblems, limitations and disadvantages of prior techniques of NOxreduction from exhaust gases produced by a variety of industrialprocesses.

It is another object of the present invention to provide practical andlow-cost methods of NOx removal in a variety of industrial processes,which may utilize a variety of commercially-available reductants.

It is another object of the present invention to provide practical andlow-cost methods is of NOx removal in a variety of industrial processes,which may deplete NH₃ and CO in a substantially concurrent manner.

It is yet another object of the present invention to provide NOxreduction methods which do not require solid catalytic surfaces orhazardous materials.

It is a further object of the present invention to provide methods ofNOx removal that may be selective and conducted nearlystoichiometrically in the gas phase.

Finally, it is an object of the present invention to provideautothermal, autocatalytic NOx reduction methods in a wide variety ofindustrial processes, and also to various systems and apparatus forcarrying out the same.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned features, objects, and attendant advantages of thepresent invention may be better appreciated and understood whenconsidered in connection with the accompanying drawings, wherein:

FIGS. 1(a) to 1(c) are graphs reproduced from “Chemical ReactionEngineering” by Octave Levenspiel, John Wiley and Sons, Inc., 1962, p.228 (Library of Congress Catalog Card Number: 62-15185), and which serveto illustrate distinguishing characteristics of self-sustainedautothermal reactions as are utilized in accordance with embodiments ofthe present invention:

FIG. 2 illustrates the effects of NH₃ and HNCO on the autoignition andautothermal heat release of hydrocarbon(s) under fuel-lean conditions inan exhaust gas initially containing about 10% O₂;

FIG. 3 illustrates the selective reduction of NOx by HNCO whileintroduced hydrocarbon(s) autoignite and autothermally heat an exhaustgas containing in the range of about 5-9% O₂;

FIG. 4 illustrates the substantial concurrent depletion of CO and NH₃ byself-sustaining autocatalytic reactions in an exhaust gas in accordancewith embodiments of the present invention, wherein hydrocarbon(s)autoignite and autothermally heat the exhaust gas under fuel-leanconditions containing in the range of about 5-9% O₂;

FIG. 5 illustrates increasing depletion of CO in the presence of NH₃ athigher temperatures in the range of about 1385-1425° F.;

FIG. 6 compares the selective reduction of NOx using cyanuric acidpowder and aqueous urea solutions as alternative reductant(s) forgenerating NH₃ and HNCO to reduce NOx autocatalytically in accordancewith embodiments of the present invention.

FIG. 7 is a diagram illustrating embodiments of the present invention inwhich hydrocarbon(s) are introduced to autoignite and autothermally heatan exhaust gas while reductant(s) are introduced to generate NH₃, HNCOor an combination thereof so that NOx is reduced selectively while COand NH₃ are both depleted substantially, as may be conducted in asingle-stage treatment under fuel-lean conditions wherein the exhaustgas contains at least about 1% O₂;

FIG. 8 is a diagram illustrating an embodiment of the present inventionwherein a single-stage exhaust gas treatment such as illustrated in FIG.7 is incorporated with the production of the exhaust gas by combustionequipment, and may additionally include heat recovery, preheating or O₂enrichment in embodiments in which the present invention serves tocontrol NOx emissions while producing petrochemical products, generatingelectricity or operating machinery or mobile equipment.

FIG. 9 is a diagram illustrating an embodiment of the present inventionwherein a single-stage exhaust gas treatment such as illustrated in FIG.7 is incorporated with a heat exchange boiler so that heat released inimplementing the present invention to control NOx emissions is recoveredin conjunction with the production of petrochemical products or thegeneration of electricity by the heat exchange boiler, and thecombustion of fuel in the heat exchange boiler may additionally bemodified to control initial NOx and O₂ composition of the exhaust gasfor implementing the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described in greater detail withreference to particular preferred and alternative embodiments. Suchdescription is for a more complete understanding of the background,utility and application of the present invention, and is without beingbound by any particular theory or the like.

Referring to FIGS. 1(a) to 1(c), the material balance curves illustratethe characteristic relationship between temperature and conversion ofreactants for exothermic, irreversible reactions. The alternative energybalance lines in FIGS. 1(a), (b), and (c) illustrate the potentialadiabatic heat release starting from the same initial temperature T. Aswill understood in the art, the actual conversion and heating depend onsatisfying the material and energy balances simultaneously asrepresented by the points of intersection, which may described asfollows.

In FIG. 1(a), the initial temperature and amount of reactants areinsufficient for self-sustained adiabatic heating beyond the point ofintersection. The extent of reaction remains negligible because theheating required to sustain the reaction exceeds the potential amount ofheat release from the reaction. In FIG. 1(b), the larger amount of heatrelease enables practically complete conversion starting from the sameinitial temperature. In this case, a high final temperature necessarilyresults from the high heat release starting at a temperature lower thanthe ignition temperature as described below. FIG. 1(c) illustrates thesignificance of ignition as a limiting condition for self-sustainedadiabatic heating using less heat release. At any initial temperatureless than the ignition condition, the heating required to sustain thereaction exceeds the heat release, so the reaction can never proceedadiabatically beyond point M′, similar to FIG. 1(a). Appreciableconversion is self-sustained only when the reactants are first ignited;then, the same heat release may self-sustain nearly complete conversionas illustrated by point M′″.

Hydrocarbon(s) and CO are known to oxidize by exothermic, irreversiblereactions. The heat release from these reactions may self-sustain theconversion of fuels in combustion equipment. Such combustion usingexcess air produces exhaust gases at high temperatures where thereactant fuel and O₂ are typically preheated above the ignitiontemperature using a stabilized flame. Such hot oxidizing gases, however,generate NOx emissions thermally and convert fuel nitrogen, includingNH₃ and HNCO, to form NOx.

Such hot oxidizing conditions are avoided by limiting the conversion ofreactants in previous teachings related to noncatalytic NOx reduction.According to such teachings, the conversion is limited by a deficiencyin either O₂ or the ancillary reducing material. Referring again toFIGS. 1(a) to 1(c), it may be understood that either of thesedeficiencies limits both the heat release and the self-sustainingconversion of reactants. The limited conversion, however, tends tonecessarily result in the production of air pollutants as describedpreviously.

Such teachings, for example, generally correspond to the introduction ofNH₃, HNCO or hydrocarbon(s) into an 0₂-deficient, fuel-rich exhaust gasor the use of stratified fuel mixtures to generate partial oxidationproducts. In accordance with such teachings, the partial oxidationproducts reduce NOx directly under O₂-deficient, fuel-rich conditions orlower the effective temperature for selective NOx reduction by NH₃ andHNCO in the presence of excess O₂.

As previously explained, according to these teachings: “Where it isdesired to lower the operating temperature to a greater degree, largeramounts of CO or other H atom generating species will be added orvice-versa.” As discussed in connection with FIGS. 1(a) to 1(c),however, larger amounts of ancillary reducing materials must beconverted incompletely in order to lower the operating temperature.Similarly, the reported inhibition of CO oxidation by NH₃ and HNCO mustnecessarily decrease the adiabatic heating of an exhaust gas containingexcess O₂.

According to such teachings, the production of byproduct air pollutantsis avoided only by removing CO prior to the injection of NH₃ or HNCO.Such teachings, however, tend to require higher temperatures in therange of 1600-2000° F. for noncatalytic NOx reduction unless hydrogen isused as the ancillary reducing material. Although hydrogen may lower theeffective temperature to reduce NOx noncatalytically without producingother air pollutants, this technique is limited as previously described.

Such previous teachings do not appear to appreciate the possibility ofan ignition condition for self-sustaining gas-phase reactions to reduceNOx selectively. Contrary to such previous teachings, NH₃, HNCO or acombination thereof may be utilized for effective autocatalytic NOxreduction when hydrocarbon(s) autoignite and heat an exhaust gasautothermally under fuel-lean conditions wherein the exhaust gascontains at least about 1% O₂. Such unique features, aspects andattributes of the present invention will become more apparent to thoseskilled in the art by the following discussion, referring to the figuresand tables discussed below.

FIG. 2 illustrates the autothermal heating of an exhaust gas containingabout 10% O₂ and 1555 ppm NOx at an initial temperature of about 920° F.The exhaust gas was produced by a heavy duty high speed diesel engine.The solid line in FIG. 2 shows the temperature profile when diesel fuelalone was injected to autoignite and autothermally heat the exhaust gas.Temperatures measured using stainless-sheathed type K thermocouplesalong the gas flowpath never reached 1400° F. when diesel fuel wasinjected alone. The final exhaust gas composition contained about 7.6%O₂, and the initial NOx level was not reduced appreciably by the dieselfuel injection at this high O₂ level.

The solid rectangles in FIG. 2 illustrate the measured temperatures whencyanuric acid was injected in addition to the diesel fuel. The cyanuricacid was injected to decompose and generate HNCO at a molar ratio ofabout 1.4 with respect to the initial NOx in the exhaust gas. The lowertemperatures in comparison with diesel fuel alone at residence timesless than about 0.4 seconds indicate that the introduction of HNCOslowed the initial autothermal heating of the exhaust gas using the sameamount of diesel fuel. The additional introduction of HNCO eventuallyresulted in a higher temperature in excess of 1400° F. after about 0.4seconds, indicating more autothermal heating than the diesel fuel alone.In addition, the NOx emissions were reduced more than 90%, and the finalCO concentration increased from about 185 ppm to about 520 ppm.

As illustrated in FIG. 2, the introduction of HNCO increased the finaltemperature for autothermal heating of the exhaust gas in comparisonwith injecting the same amount of diesel fuel alone. Since the HNCOcontributed over 2000 ppm of reactant CO, the much smaller increase ofonly 335 ppm in the final CO concentration demonstrated a greaterconversion of CO in comparison with the same introduction of diesel fuelalone. These results demonstrate a uniquely greater conversion of bothNOx and CO concurrently in accordance with the present invention.

The initially lower temperatures with HNCO generation illustrated inFIG. 2 greatly exceed any cooling effects related to the decompositionand vaporization of cyanuric acid. These lower temperatures reflect aninitially slower rate of heat release from the diesel fuel due to thegenerated HNCO. This initial inhibition of the autothermal heating didnot last more than about 0.4 seconds. After this initial inhibition, theexhaust gas was heated faster to temperatures above about 1400° F., andboth the NOx and CO were substantially depleted as a result of the HNCOgeneration during the autothermal heating.

These experimental results support a uniquely autocatalytic NOxreduction distinctly different from the previous teachings. As describedby Levenspiel, autocatalytic reactions are distinguished by theself-sustained conversion of reactants which initially starts slowly asillustrated in FIGS. 1 and 2. In these reactions, at least one reactionproduct acts as a catalyst. The reactions proceed faster with formationof the catalyst and continue until reactants are depleted substantially.

When NOx emissions are reduced autocatalytically, reductant(s) andhydrocarbon(s) are both substantially converted by reactions whichrelease heat autothermally as illustrated in FIGS. 1 and 2. The exhaustgas temperature is increased by adding either more reductant(s) or morehydrocarbon(s). This is contrary to the previous teachings where largeramounts of ancillary reducing materials are added to lower thetemperature for NOx reduction or where reductant(s) are added underO₂-deficient, fuel-rich conditions, etc.

Similar single-stage experiments were performed using an exhaust gasfrom another diesel engine. This exhaust gas was preheated in the rangeof about 1100-1300° F. by the combustion of supplemental fuel. Thesupplemental fuel was combusted completely so that the preheated engineexhaust gas contained virtually no CO, and about 8.7% O₂ and 700 ppmNOx.

The NOx emissions were reduced autocatalytically by introducing dieselfuel and cyanuric acid after the exhaust preheating. By preheating theexhaust gas, less diesel fuel was needed for autoignition andautothermal heating to final temperatures in the range of about1385-1425° F. Using less diesel fuel in comparison with the resultsillustrated in FIG. 2 actually increased the selectivity of NOxreduction as illustrated in FIG. 3.

FIG. 3 illustrates the selectivity and levels of NOx reduction in thesesingle-stage autocatalytic treatments when the exhaust gas was preheatedin the range of about 1240-1270° F. The initial NOx level of about 700ppm was reduced as much as about 95% to levels as low as about 31 ppm.The NOx was reduced about 80-90% nearly stoichiometrically, and thisnearly 100% selectivity declined only gradually when more HNCO was addedto reduce NOx further.

For molar ratios of HNCO with respect to NOx in the range of about0.8-1.6, the CO and NH₃ were depleted in a substantially concurrentmanner as illustrated in FIG. 4. The extent of this concurrent depletionwas increased by introducing more diesel fuel so that the exhaust gaswas heated autothermally to a higher temperature in the range of about1385-1425° F. This trend with temperature is illustrated in FIG. 5.

Contrary to previous teachings, the amount of diesel fuel did not changethe operating temperature for selective NOx reduction. The autothermalheating of about 480° F. in FIG. 2 was decreased in the range of about100-300° F. for the experimental results illustrated in FIGS. 3-5. Thesubstantial depletion of CO and NH₃ confirmed nearly complete conversionof reactants, and the final operating temperature for NOx removalremained near 1400° F.

As may be understood by the discussion in connection with FIGS. 1(a) to1(c), autoignition of reactants enables nearly complete conversion atabout the same final temperature, regardless of the initial temperature.Higher initial temperatures above the ignition condition decrease theautothermal heat release that is needed for self-sustaining nearly thesame conversion of reactants. The aforementioned comparisons of theexperimental results in FIGS. 2-5 are indicative of the characteristicsof autocatalytic reactions.

Autoignition and autothermal heating represent requirements/conditionsto reduce NOx autocatalytically in accordance with the presentinvention. Without these key conditions, NH₃ and HNCO reduce NOxnoncatalytically. The previous teachings for noncatalytic NOx reductionpreclude any single-stage method for overcoming the limiting inhibitionof CO oxidation by NH₃ and HNCO, as may be understood from the previousdiscussion or a review of such prior teachings. The experimental resultsprovided herein, however, clearly demonstrate different conditions ofautoignition and autothermal heating advantageously utilized inaccordance with the present invention to reduce NOx autocatalytically.

These inventively new conditions for autocatalytic NOx reduction do notdepend on solid surfaces to catalyze the reactions. Contrary to previousteachings, the final operating temperature for the gas-phase,autocatalytic reactions does not change in general with the use ofalternative reductant(s) or different amounts of hydrocarbon(s). Thesecharacteristics of autocatalytic NOx reduction in accordance with thepresent invention are further evidenced by the comparison of testresults in Table 1 below.

TABLE 1 Test 1 Test2 Test 3 Test4 Initial Diesel Engine ExhaustTemperature, ° F. 934 1243 1281 1316 NOx, ppm 1550 700 700 700 FinalTreated Exhaust Temperature, ° F. 1444 1471 1429 1482 O₂, % 8.0 6.2 7.84.5 NOx, ppm 64 46 52 54 CO, ppm 98 18 93 54 Reductant (Source of HNCO)Mix CYA CYA Urea Molar Ratio (HNCO/NOx) 1.1 1.1 1.1 0.8 ReactionResidence Time, sec 0.7 0.4 0.7 0.4 Reaction Chamber Lining SS SS CF CFSS = Stainless Steel; CF = Ceramic Fiber

Test 1 was performed using the same exhaust gas and reaction chambercorresponding to the results illustrated in FIG. 2. This reactionchamber is lined with stainless steel and contains stainless steelbaffles. Tests 2-4 were performed using the same exhaust gas andreaction chamber corresponding to the results illustrated in FIGS. 3-5.This reaction chamber is lined with ceramic fiber insulation andcontained stainless steel internals which were removed only for Tests 3and 4. Cyanuric acid powder (CYA) was injected pneumatically in Tests 2and 3 as the source of HNCO. An aqueous solution containing about 25% byweight urea was injected as the source of HNCO in Test 4. A powderedproduct of urea decomposition (Mix) was injected pneumatically inTest 1. This mixed source of HNCO was estimated to contain about 25%cyanuric acid along with 75% biuret and urea for calculating the molarratio of HNCO with respect to NOx.

Tests 1-4 were all performed nearly adiabatically using diesel fuelinjection to control the final temperature of the autothermal heating.The injection of diesel fuel was decreased in Tests 2-4 where the engineexhaust was preheated prior to the introduction of the diesel fuel andreductant(s). The final O₂ concentrations reflect differences in thepneumatic conveying of powdered reductant(s) and the exhaust gaspreheating as well as the amount of diesel fuel injected to autoigniteand heat the exhaust gases autothermally.

Despite the differences in exhaust gas composition and preheating,reaction chamber materials and residence times, and the chemicalcomposition of reductant(s), the NOx and CO emissions were alwaysreduced in a substantially concurrent manner. Higher final temperaturesin the narrow range of about 1400-1500° F. converted the reactants tosimilarly low levels over a wide range of initial NOx concentrations andautothermal heat releases.

As illustrated by the data in Table 1, higher final temperatures for theautothermal heating primarily depleted the reactants in shorterresidence times. This depletion of reactants included NH₃ even when theNH₃ was generated in large amounts by the thermal decomposition of urea.In fact, the results of Test 4 demonstrate NOx reduction in excess ofthe HNCO generated from urea, indicative that high NH₃ levels may beeffective for reducing NOx autocatalytically in accordance with thepresent invention.

The comparisons in Table 1 are based on the molar ratio of HNCO withrespect to NOx because of the uncertainty in estimating the generationof NH₃ from the mixture of urea decomposition products. This uncertaintyis removed when urea and cyanuric acid are compared directly in FIG. 6.For this comparison, the molar ratios are based on generating equalamounts of NH₃ and HNCO from urea, as reported in Combust. Sci. andTech., Vol. 65, 1989.

On this basis, NH₃ reduces NOx autocatalytically less selectively thanHNCO as illustrated in FIG. 6. The data from FIG. 3 are repeated in FIG.6 for direct comparison of the similar experiments. The urea was testedin place of the cyanuric acid by injecting an aqueous solutioncontaining about 25% by weight urea. While the generated mixture of NH₃and HNCO reduced NOx less selectively than HNCO alone, the NOx wasreduced autocatalytically to the same low level using urea or cyanuricacid interchangeably.

An independent source test contractor analyzed the engine exhaust gasboth before and after Test 1 in Table 1. These analyses for typicalcombustion byproducts are summarized in Table 2 below. As illustrated bythese results, autocatalytic NOx reduction in accordance with thepresent invention does not produce any significant amount of typicalcontaminants in combustion exhaust. In fact, the autothermal reactionswith autocatalytic methods for NOx reduction as provided herein mayactually lower other typical exhaust contaminants besides NOx.

TABLE 2 Baseline Test 1 Engine Byproduct Exhaust Results CO, ppm 210 98CH4, ppm 3.2 2.6 HCN, ppm 0.079 0.087 Solid PM, gr/dscf 0.0099 0.0096Total PM, gr/dscf 0.0167 0.0139 organics, ppm 0.5 0.5 (non-CH₄)

Similar results were obtained in applying autocatalytic methods of NOxreduction in accordance with the present invention to preheated exhaustgas from a 4000 bhp medium speed diesel electric generator with acapacity of about 2.8 MW. In this case, the engine exhaust was producedat temperatures below about 600° F. and was preheated above about 1000°F. using a heat exchanger.

The final temperature of the autocatalytic NOx reduction was maintainedat about 1400° F. by controlling the introduction of diesel fuel in thepreheated exhaust. The heat exchanger cooled the treated exhaust bypreheating the engine exhaust. Supplemental fuel was fired in a burnerto preheat the engine exhaust initially as a means for starting the heatrecovery.

The preheated exhaust was introduced to the reaction chamber through around duct with a 36-inch inside diameter. The diesel fuel and powderedcyanuric acid were introduced into this duct. The diesel fuel wasdispersed in the exhaust gas using pressure atomization from a singleliquid spray nozzle. The cyanuric acid powder was introducedpneumatically.

In this particular example, the preheated engine exhaust contained morethan about 13.5% O₂ at temperatures above about 1000° F. Under theseconditions, stratified ignition was prevented by swirling the entireflowrate of exhaust gas which totaled nearly 13,000 dscfm. Uniformautoignition and autothermal heating were verified visually by theappearance of a chemiluminescence with a substantially uniform bluecolor as the exhaust gas entered the open reaction chamber.

The reaction chamber consisted of a round open vessel lined with ceramicfiber insulation. The inside diameter of the insulation lining was about7 feet. The blue chemiluminescence was observed to appear in the largeopen gas space. The exposed surfaces in the large open vessel werenegligible for catalyzing any reactions. The residence time forgas-phase reactions in the open vessel was about 1.3 seconds.

Emissions source tests were performed by an independent contractor.These measurements confirmed the previous test results as illustrated inTable 3. At the relatively long residence time of about 1.3 seconds, theNH₃ breakthrough was depleted below about 2 ppm, while the CO wasdepleted below about 50 ppm at the final temperature of about 1400° F.In addition, the particulate emissions were decreased concurrently bynearly 70%.

TABLE 3 4000 bhp 2.8 MW baseline treated exhaust exhaust Engine Fuel,lb/hr 1347 1368 Engine Exhaust, ° F. 559 599 Preheated Exhaust, ° F.1046 Final Reaction, ° F. 1400 Cyanuric Acid, lb/hr 79.1 ExhaustFlowrate, dscfm 12,974 12,970 O₂, % 13.78 11.5 CO₂, % 5.15 7.0 NOx (15%O₂), ppmc 481 45.7 CO, ppm 47.0 49.2 NH₃, ppm 1.8 Particulates, gr/dscf0.0196 0.0061

The treated NOx levels were measured before the cooled exhaust wasdischarged to the atmosphere. The rate of cyanuric acid introduction wascontrolled to maintain a treated NOx level. This NOx level wasmaintained over a range of engine loads based on both NOx and loadmeasurements. The NOx emissions were maintained below a regulatorycompliance level using these measurements either individually orcollectively.

These consistent results using different reactor configurations confirmthe characteristic features of NOx reduction by gas-phase autocatalyticreactions in accordance with the present invention. Contrary to previousteachings, all of the reactants including NH₃, HNCO, CO andhydrocarbon(s) may be depleted in a substantially concurrent manner in asingle-stage treatment. Also, autocatalytic treatment in accordance withthe present invention may substantially reduce levels of typicalcontaminants in combustion exhaust gases.

The concurrent depletion of reactants clearly is indicative of anautocatalytic mechanism as described by Levenspiel. The key ignitioncondition for self-sustained autocatalytic reactions is supportedvisually by the appearance of a substantially uniform bluechemiluminescence. Contrary to previous teachings, such autocatalyticreaction conditions in the presence of excess O₂ do not necessarilyoxidize NH₃ or HNCO to form NOx.

The experimental results produced by Applicants indicate that NH₃, HNCOor a combination thereof may be effective for autocatalytic NOxreduction in accordance with the present invention when hydrocarbon(s)autoignite and heat an exhaust gas autothermally under fuel-leanconditions wherein the exhaust gas contains at least about 1% O₂. Undersuch conditions, reactants including NOx, NH₃, HNCO, CO, hydrocarbon(s)and other typical exhaust contaminants may be reduced in a substantiallyconcurrent manner.

This concurrent conversion of reactants conflicts with previousteachings for noncatalytic NOx reduction. According to these teachings,CO oxidation is limited by the inhibiting effects of NH₃ or HNCO. Thislimiting conversion of reactants is previously described to produce airpollutants unless hydrogen is used as the ancillary reducing materialfor lowering the effective temperature to reduce NOx selectively, etc.

In accordance with the present invention, both NH₃ and CO may bedepleted together when the autocatalytic reactions are self-sustained toa final temperature in the range of about 1400-1550° F. Unexpectedly,NH₃ may be depleted below even 2 ppm concurrently with CO removal belowabout 50 ppm. The concurrent removal of NH₃ and CO is illustrated inFIG. 4 and is unique to the autocatalytic reaction method. Based on thischaracteristic relationship, CO measurements may be used to reliablyindicate the level of NH₃ breakthrough, and in certain preferredembodiments a CO measurement is used in an industrial process todetermine whether NH₃ breakthrough is occurring and/or is exceeding apredetermined level.

Higher final temperatures in the range of about 1400-1550° F. primarilydeplete the reactants autocatalytically in a shorter residence time whenthe exhaust gas is preheated above about 900° F. A maximum residencetime of about 1.5 seconds is decreased below about 1.0 seconds bypreheating the exhaust gas above about 1050° F. Also, the residence timeis decreased below about 0.5 seconds by preheating the exhaust gas aboveabout 1200° F.

The autocatalytic reactions described herein, unexpectedly, may beeffective for reducing NOx selectively when the exhaust gas is heatedautothermally to the temperature range of about 1400-1550° F. This rangeof final temperatures is not highly dependent on the amount ofhydrocarbon(s) introduced to autoignite and heat the exhaust gasautothermally. This conflicts with the previous teachings, where largeramounts of ancillary reducing materials are added to lower the effectivetemperature for NOx reduction by NH₃ and HNCO.

In accordance with the present invention, emissions of NOx may bereduced autocatalytically using NH₃, HNCO or a combination thereof. TheNOx emissions are reduced about 80-90% to about 50-200 ppm using NH₃ andHNCO nearly stoichiometrically. Furthermore, NOx emissions may bereduced by as much as 99% to levels as low as about 10 ppm using no morethan about twice the stoichiometric ratio of NH₃ and HNCO with respectto NOx. The NOx emissions may be reduced selectively using a molar ratioof NH₃ and HNCO together with respect to NOx in the range of about0.5-2.0.

Contrary to previous teachings, cyanuric acid, urea and NH₃ may be usedin a substantially interchangeable manner for reducing NOxautocatalytically. When hydrocarbon(s) autoignite and heat the exhaustgas autothermally to temperatures in the range of about 1400-1550° F.,reductant(s) which generate(s) NH₃, HNCO or a combination thereof may beeffective for reducing NOx to a low level in accordance with the presentinvention.

Uniform distribution of NOx, hydrocarbon(s) and reductant(s) ispreferred to maintain fuel-lean conditions for the autoignition,autothermal heating and selective NOx reduction. Contrary to previousteachings, stratified fuel mixtures or other methods of stagingfuel-rich combustion do not benefit selective NOx reduction. Theseprevious teachings appear to adversely extinguish the conversion ofreactants, which tends to result in the production of other airpollutants in addition to decreasing the selectivity of NOx reduction incomparison with the new, single-stage autocatalytic methods describedherein.

As illustrated in FIG. 7, autocatalytic methods in accordance with thepresent invention may be implemented in a single stage. Thehydrocarbon(s), reductant(s) and exhaust gas may be mixed together forautoignition, autothermal heating and selective NOx reduction.Preferably, the exhaust gas should contain about 2-18% O₂ at atemperature in the range of about 900-1600° F. for the spontaneousignition and autothermal heating to deplete the generated reactantssubstantially. The wavy arrows in FIG. 7 represent the mixing or otherdispersion of reactants to maintain fuel-lean conditions wherein theexhaust gas contains at least about 1% O₂.

The exhaust gas mixing may start before or after the introduction ofhydrocarbon(s) and reductant(s), and the mixing may extend throughout aportion, or even all, of the autothermal heating. Baffles or swirl vanesmay modify the exhaust gas flowpath to mix the reactants. One or morespray nozzles may disperse the hydrocarbon(s) and reductant(s)substantially throughout the cross-section of the exhaust gas flowpath.Preferably, the mixing, dispersion or combination thereof shouldestablish a substantially uniform exhaust gas composition before theappearance of a chemiluminescence.

In accordance with the present invention, the reductant(s) may comprisematerial(s) selected from the group consisting of NH₃, HNCO, urea,decomposition products of urea, cyanuric acid or a tautomer of cyanuricacid, compounds which decompose to produce NH₃ as a byproduct, ammoniumsalts of organic acids, hydrocarbon amines or combinations of theforegoing, whether pure compounds or mixtures, as solids, liquid melts,emulsions, slurries or solutions in water, alcohols, hydrocarbons oroxygenated hydrocarbon solvents.

In general, only the selectivity of NOx reduction and the conversion ofNOx to N₂O depend significantly on the chemical reductant(s). While NOxreduction by NH₃ generates less byproduct N₂O emissions, HNCO removesNOx more selectively. In either case, preferably a molar ratio of NH₃and HNCO together in the range of about 0.5-2.0 with respect to NOx maybe utilized for effective reduction of NOx as much as 99% to levels inthe range of about 10-200 ppm.

The introduction of reductant(s) preferably is controlled to maintain alevel of NOx reduction or a final NOx emissions level in the treatedexhaust gas. In certain preferred embodiments, continuous measurementsof NOx emissions are used to increase or decrease the introduction ofreductant(s) as a part of a feedback control system for maintaining afinal NOx emissions level (set at a desired, predetermined level, forexample). Alternatively, NOx emissions source tests can establishcharacteristic relationships between operating conditions for thecombustion equipment and the introduction of reductant(s) needed for adesired level of NOx reduction. Based on these relationships, thecontinuous monitoring of operating conditions can be used forfeedforward control of the NOx emissions reduction. Also, combinationsof feedback and feedforward control can reliably maintain final NOxemissions levels despite variations in the exhaust gas flowrate andbaseline NOx levels during combustion operations.

Hydrocarbon(s) utilized in accordance with embodiments of the presentinvention may comprise material(s) selected from the group consisting ofhydrocarbon mixtures such as natural gas, liquefied petroleum gas,alcohols, gasoline, diesel fuel, aviation turbine fuel, variousoxygenated hydrocarbons, hydrocarbon amines or any fraction of suchmixtures, including purified components such as carbon monoxide,methane, propane, methanol, and ethanol, whether introduced as liquidsor vapors. In addition, the hydrocarbon(s) may include the same liquid,gaseous or vaporous fuels that are combusted to produce the exhaust gascontaining NOx.

Autocatalytic methods in accordance with the present invention ingeneral do not depend on the order in which the reductant(s) andhydrocarbon(s) are introduced into the exhaust gas. Within thetemperature range of about 900-1600° F., the introduced hydrocarbon(s)autoignite spontaneously under fuel-lean conditions of about 2-18% O₂ inthe exhaust gas. The NH₃, HNCO or combination thereof may be introducedor generated from reductant(s) at any time before or during thefuel-lean autothermal conversion of hydrocarbon(s) and CO in the exhaustgas.

Self-sustaining autothermal reactions in the gas phase as utilizedherein tend not to be adversely affected by other exhaust gascontaminants, including gaseous organics, particulate matter or CO. Infact, such autothermal reactions may serve to at least partially removethese typical exhaust contaminants. This conversion of contaminants mayreduce objectionable emissions while decreasing the amount ofhydrocarbon(s) required to maintain a final temperature for depleting COand NH₃ breakthrough in a substantially concurrent manner.

In preferred embodiments, the introduction of hydrocarbon(s) iscontrolled to maintain a final temperature in the range of about1400-1550° F. The autothermal heat release increases the exhaust gastemperature adiabatically in the absence of heat losses, or inalternative embodiments heat transfer surfaces may recover heat from theexhaust gas during the autothermal heating. In general, the heatrecovery should not cool the exhaust gas so excessively as to extinguishthe autothermal reactions before the completion of NOx removal anddepletion of CO and NH₃ breakthrough.

An autothermal heat release equivalent to an adiabatic temperatureincrease in the range of about 50-500° F. is utilized in preferredembodiments to achieve a final exhaust gas temperature in the range ofabout 1400-1550° F. for implementing such autocatalytic methods. Theamount of hydrocarbon(s) introduced depends primarily on the initialexhaust gas temperature and any recovery of heat released by theautothermal reactions.

Autocatalytic methods in accordance with preferred embodiments typicallyrequire residence times no longer than about 1.5 seconds. In general, COand NH₃ are depleted faster when the autothermal heating is initiated athigher temperatures in the range of about 1050-1600° F. With suchembodiments, reaction residence times in the range of about 0.02-1.0seconds may be sufficient to deplete both CO and NH₃ substantially.Still higher initial exhaust gas temperatures in the range of about1200-1600° F. enable substantial CO and NH₃ depletion within the rangeof about 0.02-0.5 seconds.

The introduction of hydrocarbon(s) decreases beneficially when theexhaust gas is preheated to the temperature ranges of about 1050-1600°F. or about 1200-1600° F. In these cases, the autothermal heat releaseneed not exceed an amount equivalent to an adiabatic increase of about50-350° F. or 50-200° F., respectively, so long as the exhaust gas isheated autothermally to a final temperature in the range of about1400-1550° F. This preheating of the exhaust also improves theselectivity of NOx reduction.

In general, the initial exhaust gas temperatures do not depend on howthe exhaust gas is preheated or cooled so long as the O₂ concentrationis maintained in the range of about 2-18% by volume. The exhaust gas maybe heated or cooled initially using heat transfer surfaces, includingany of various methods for preheating the exhaust gas by recovering heatafter the exhaust gas is treated using autocatalytic methods as providedherein. Alternatively, the exhaust gas may be heated directly by thecombustion of a supplemental fuel in the exhaust gas.

The combustion of a supplemental fuel using excess air also may serve toenrich the O₂ concentration in an otherwise 0₂-deficient exhaust gas. Inthis case, the supplemental fuel combustion can serve the dual purposeof preheating the exhaust gas and enriching its O₂ concentration. Thecombustion of a supplemental fuel also may serve to preheat a portion ofthe exhaust gas to ignite more supplemental fuel which is combusteddirectly in the exhaust gas. If the exhaust gas is preheated usingfuel-rich combustion, autocatalytic methods as provided herein maypartially remove additional contaminants from the fuel-rich combustion.

Autocatalytic methods in accordance with the present invention may beused in combination with various modifications to the combustion processfor producing the exhaust gas. Such modifications may beneficially lowerNOx emissions and maintain exhaust gas temperatures within the range ofabout 900-1600° F., or preferably about 1200-1600° F., for autocatalyticmethods as provided herein, so long as the O₂ concentration remainsabove about 2% by volume. Such modifications also may beneficiallydecrease the introduction of both reductant(s) and hydrocarbon(s) insuch autocatalytic methods.

Autocatalytic methods such as illustrated in FIG. 7 are implemented inalternative embodiments using any of the various techniques forincorporating the exhaust aftertreatment with combustion equipment asillustrated in FIGS. 8 and 9. Autocatalytic methods in accordance withthe present invention also may be implemented in conjunction with theprimary combustion process so that the autothermal heat release isrecovered beneficially as illustrated in FIG. 9. For example, existingor new surfaces in a heat exchange boiler are used in certainembodiments to also recover the autothermal heat release produced withautocatalytic methods as provided herein to replace primary fuel, andmay be used, for example, for the purpose of generating steam and/orelectricity, or cracking petrochemicals, or the autothermal heating mayserve to increase the generating capacity of an existing boiler, etc.,as illustrated. In such embodiments, mechanical work is conducted,petrochemicals are cracked or otherwise processed, steam and/orelectricity is generated, heat recovered etc., which may be conductedunder conditions that in general are optimized for the primary process.Use of autocatalytic methods in accordance with the present inventioneffectively enables the NOx reduction to be dissociated from the primaryprocess, and thereby enabling the primary process to be conducted in amore optimum manner.

Combustion modifications such as over-fire air may be used to enablelower NOx emissions from the primary fuel while also enriching O₂ in theexhaust gas. In the case of coal-fired boilers, the replacement ofprimary fuel by autothermal heating may serve to increase furnace O₂levels beneficially for the purpose of decreasing unburned carbon on flyash. Such benefits of autocatalytic methods as provided herein may serveto increase overall boiler efficiency, while also enhancing the value ofbyproduct fly ash, possibly avoiding the generation of an otherwisesolid waste.

Autocatalytic methods in accordance with the present invention also maybeneficially consume NH₃ breakthrough from a previous exhaust gastreatment using SNCR. In such embodiments, the autocatalytic methods mayreplace the use of SCR as a means for controlling NH₃ breakthrough fromSNCR. Autocatalytic methods, however, may be preferably applied in placeof SNCR in order to reduce NOx more selectively. The better selectivityof autocatalytic methods as provided herein may greatly decrease theconsumption and cost of reductant(s), while depleting both CO and NH₃ ina substantially concurrent manner, and reducing NOx emissions to lowerlevels.

Autocatalytic methods in accordance with the present invention mayreduce NOx emissions below most regulatory requirements, and the presentinvention typically supplants the need for expensive catalystsaltogether. Autocatalytic activity for removing NH₃ and CO along withNOx in accordance with the present invention may be self-sustained andin a manner that does not deteriorate with use like solid catalysts. Asa result, autocatalytic methods as provided herein may avoid the need toreplace existing catalysts poisoned by exhaust contaminants.

If emissions regulations require additional NOx reductions, thenautocatalytic methods as provided herein may serve to enhance SCRapplications while minimizing the catalyst volume. In addition toreducing NOx before SCR, autocatalytic methods also may decreasecontaminants such as hydrocarbons and soot which may foul catalyticsurfaces. In embodiments utilizing such combination of treatments,autocatalytic methods may enable the use of more efficient orcost-effective catalyst beds due to both contaminant removal and thecontrol of exhaust gas temperatures.

In a preferred combination of autocatalytic and catalytic reductionmethods, the autothermal heating continuously decreases hydrocarbon andsoot contaminants while controlling the exhaust gas temperature to thecatalyst. Emissions of NOx are maintained at the required level using aseparate injection of NH₃ ahead of the catalyst to minimize reductant(s)utilized with the autocatalytic method. In many such situations, theautocatalytic NOx reduction need not exceed about 80-90%, soreductant(s) are converted nearly stoichiometrically and CO emissionsare depleted substantially in the shortest time possible.

Substantial depletion of both NH₃ and CO uniquely enables autocatalyticreduction of nonuniform NOx and CO distributions. Similar to catalyticmethods, autocatalytic methods may remove excess NH₃ while reducing NOxto low levels. As a result, nonuniform distributions of NOx in theexhaust gas tend to only decrease the selectivity of NOx reduction bythe autocatalytic methods. By reducing both NOx and NH₃ to uniformly lowlevels, autocatalytic methods may replace the need for mixing theexhaust gas, especially when such autocatalytic methods are combinedwith SCR to achieve ultra-low emissions levels.

Autocatalytic methods as provided herein are uniquely suited forcombination with catalytic air heater elements. Such catalytic elementsreplace existing heat transfer surfaces in rotary regenerative airheaters, such as described in U.S. Pat. Nos. 4,602,673, 4,678,643,4,719,094 and 4,867,953. Using autocatalytic methods to controltemperature as well as uniform NOx levels ahead of the catalyst greatlyenhances the capability for both NOx and NH₃ removal within the limitedvolume for the installation of catalytic elements in existing airheaters.

In heat exchange boiler embodiments, replacement of air heater elementsalso may enhance the recovery of heat released with autocatalyticmethods. Increasing exhaust gas temperatures enhances heat recovery byall existing surfaces downstream from the heat release, but thisenhancement does not generally recover all of the autothermal heatrelease. Complete recovery of the autothermal heat release usuallyrequires the installation of some new heat transfer surfaces.Replacement air heater elements with improved surfaces may convenientlybe utilized to complete the recovery of the autothermal heat release.

This replacement of air heater elements may uniquely complete therecovery of the autothermal heat release while also providing catalyticsurfaces to achieve ultra-low levels of both NOx and NH₃. In certainembodiments, such modifications may even enable improvements to boilerefficiency by lowering stack gas temperatures. Boiler efficiency may beimproved even further when autocatalytic methods are implemented incombination with combustion modifications to decrease the unburnedcarbon on fly ash.

In such various applications, autocatalytic methods as provided hereinare implemented by controlling the final temperature of autothermalheating at a level in the range of about 1400-1550° F. This controlledcondition preferably is achieved using one or more injection nozzles forthe introduction of hydrocarbon(s) and reductant(s). Such nozzles mayinject the hydrocarbon(s) and reductant(s) separately or concurrently,as mixtures, solutions, emulsions, slurries or combined chemicalstructures, using combinations of solids, liquids, and gases.

Furthermore, the introduction of hydrocarbon(s) and reductant(s) mayinvolve the use of pressurized gas to convey or atomize the injectedmaterial(s). The pressurized gas may consist of steam, air, exhaust gasor the gaseous or vaporous forms of the hydrocarbon(s) or reductant(s).When compressed air is used to convey or atomize any of the injectedmaterial(s), the compressed air may beneficially enrich the O₂concentration in the exhaust gas.

In certain embodiments, one or more injection nozzles are arranged invarious ways so long as the distribution of hydrocarbon(s) andreductant(s) in the exhaust gas is sufficiently uniform to avoidlocalized heating beyond the temperature range of about 1400-1550° F.The distribution and mixing of injected material(s) may include varioususes of vanes or baffles to swirl the exhaust gas or to generateturbulent mixing eddies. Such methods or their combinations shoulddisperse the injected hydrocarbon(s) in a substantially uniform mannerbefore the appearance of a chemiluminescence.

Locating such baffles or swirl vanes either before or after theinjection locations may enable the continued mixing of the exhaust gasthroughout a portion, or even all, of the flowpath during autothermalheating. Any such modifications to the gas flowpath preferably extendover the entirety of its cross-section. The use of such methods formixing the exhaust gas also may enable the introduction ofhydrocarbon(s) and reductant(s) using a variety of convenient nozzleconfigurations such as wall injectors or injection grids.

In certain embodiments, the use of injection grids may avoid the needfor modifying the gas flowpath. In such embodiments, a multiplicity ofnozzles preferably are used to distribute hydrocarbon(s) andreductant(s) over the cross-section of the gas flowpath. Such injectiongrids may resemble the well known commercial practice of distributingprevaporized NH₃ ahead of solid catalysts in SCR methods. Alternatively,the injection grids may consist of liquid nozzles using either pressureor gas atomization.

When hydrocarbon(s) or reductant(s) are introduced as liquids, thepreferred atomization depends on the degree of exhaust gas mixing. Incertain embodiments, no atomization is needed if the exhaust gas is wellmixed by turbulent flow conditions. For example, autocatalytic methodsin accordance with the present invention may be implemented in ducts aslarge as three feet or more in diameter using only a single injectorwhen the exhaust gas is swirled sufficiently to mix the vaporizedhydrocarbon(s) and reductant(s). Atomized drops in the range of about20-500 microns, however, are preferred in embodiments where the mixingdepends primarily on dispersion.

Hydrocarbon(s) and reductant(s) are preferably introduced in their mostconcentrated form. Using concentrated mixtures or solutions tends tominimize heat losses due to the sensible and latent heats of carrierliquids or gases. The dilution of injected material(s), however, mayenhance their dispersion in some cases. For example, aqueous solutionsof urea may require dilution in order to supply a multiplicity ofnozzles needed for adequate dispersion or to decrease the size ofreductant particles remaining after water evaporation from atomizeddrops. In such embodiments, commercial aqueous solutions containingabout 50% by weight urea are diluted below even a 35% concentration thatis typically used to prevent crystallization in storage tanks and otherdistribution equipment.

The distribution of hydrocarbon(s) or reductant(s) to injectors mustprevent degradation of the material(s) prior to injection. Suchdegradation can result in the plugging of distribution networks ornozzle orifices. While heating or vaporization of some material(s) mayenhance their injection without degradation, other material(s) mayrequire a means to prevent such heating or vaporization. In embodimentswhere the later may occur, the atomizing gas also may serve as a coolingmedium or insulating boundary for preventing degradation beforeinjection.

Chemical additives also may help to prevent scaling or plugging indistribution networks prior to the injection of hydrocarbon(s) orreductant(s). Such chemical additives may comprise any of thecommercially available formulations for this purpose. The use ofdistilled or de-ionized water for aqueous solutions may prevent scalingin storage tanks, piping and other equipment or instrumentation used topressurize, meter, transport, distribute and inject chemicalreductant(s) such as NH₃ or urea. In addition, scaling and corrosion areminimized by selecting proper materials of construction throughout thesesystems, depending on the injected material(s).

What may be understood from the foregoing is as follows. In accordancewith the present invention, a wide variety of methods for reducing NOxfrom industrial exhaust gases are provided, which may compriseautothermally heating the exhaust gas using one or more hydrocarbons inthe presence of NH₃, HNCO or a combination thereof, wherein theautothermal heating is under conditions effective for selectivelyreducing the NOx autocatalytically. Such methods may consist of thesteps of: controlling the initial temperature and composition of anexhaust gas in the ranges of about 900-1600° F. and about 2-18% O₂,respectively, effective for autoignition of hydrocarbon(s); controllingthe introduction of hydrocarbon(s) to autoignite and release heatautothermally effective for self-sustaining autocatalytic reactionsunder fuel-lean conditions wherein the exhaust gas contains at leastabout 1% O₂ and is heated to a final temperature in the range of about1400-1550° F.; introducing reductant(s) for NOx into the exhaust gaswherein NH₃, HNCO or a combination thereof are generated from thereductant(s); wherein NOx is reduced selectively, and the NH₃, HNCO andhydrocarbon(s), including byproduct CO are depleted substantially.

As will be appreciated from the above description, other methods inaccordance with the present invention include the following. A methodfor autothermally heating exhaust gases using hydrocarbon(s) so thatNH₃, HNCO or a combination thereof are effective for selectivelyreducing NOx autocatalytically. A method for autothermally heatingexhaust gases to temperatures in the range of about 1400-1550° F. usinghydrocarbon(s) to remove CO so that NH₃, HNCO or a combination thereofare concurrently effective for selectively reducing NOxautocatalytically. A method including the steps of: controlling theinitial temperature and composition of an exhaust gas effective forautoignition of hydrocarbon(s); controlling the introduction ofhydrocarbon(s) to autoignite and release heat autothermally effectivefor self-sustaining autocatalytic reactions under fuel-lean conditionswherein the exhaust gas contains at least about 1% O₂; introducingreductant(s) for NOx into the exhaust gas wherein NH₃, HNCO or acombination thereof are generated from the reductant(s); wherein NOx isreduced selectively, and the NH₃, HNCO and hydrocarbon(s), includingbyproduct CO are depleted substantially.

Other methods in accordance with the present invention include thefollowing. A method for autothermally heating exhaust gases usinghydrocarbon(s) so that NH₃, HNCO or a combination thereof are effectivefor selectively reducing oxides of nitrogen. A method for selectivelyreducing oxides of nitrogen in an exhaust gas using NH₃, HNCO or acombination thereof while hydrocarbon(s) autoignite and heat the exhaustgas autothermally. A method for selectively reducing oxides of nitrogenin an exhaust gas using NH₃, HNCO or a combination thereof whilehydrocarbon(s) autoignite and heat the exhaust gas autothermally,substantially depleting residual concentrations of unburnedhydrocarbons, CO, HNCO and NH₃. A method for selectively reducing oxidesof nitrogen in an exhaust gas using NH₃, HNCO or a combination thereofwhile hydrocarbons(s) autoignite and heat the exhaust gas autothermallyto a temperature in the range of about 1400-1550° F. A method forselectively reducing oxides of nitrogen in an exhaust gas using NH₃,HNCO or a combination thereof while hydrocarbons(s) autoignite and heatthe exhaust gas autothermally to a temperature in the range of about1400-1550° F., substantially depleting residual concentrations ofunburned hydrocarbons, CO, HNCO and NH₃. A method for autothermallyheating exhaust gases using hydrocarbon(s) so that NH₃, HNCO or acombination thereof are effective for selectively reducing oxides ofnitrogen while residual concentrations of unburned hydrocarbons, CO,HNCO and NH₃ are depleted substantially.

Still other methods in accordance with the present invention include thefollowing. A method for autothermally heating exhaust gases to atemperature in the range of about 1400-1550° F. using hydrocarbon(s) sothat NH₃, HNCO or a combination thereof are effective for selectivelyreducing oxides of nitrogen. A method for autothermally heating exhaustgases to a temperature in the range of about 1400-1550° F. usinghydrocarbon(s) so that NH₃, HNCO or a combination thereof are effectivefor selectively reducing oxides of nitrogen while residualconcentrations of unburned hydrocarbons, CO, HNCO and NH₃ are depletedsubstantially. A method for autoignition of hydrocarbon(s) in exhaustgases so that NH₃, HNCO or a combination thereof are effective forselectively reducing oxides of nitrogen while the exhaust gas is heatedautothermally. A method for autoignition of hydrocarbon(s) in exhaustgases so that NH₃, HNCO or a combination thereof are effective forselectively reducing oxides of nitrogen while the exhaust gas is heatedautothermally to substantially deplete residual concentrations ofunburned hydrocarbons, CO, HNCO and NH₃. A method for autoignition ofhydrocarbon(s) in exhaust gases so that NH₃, HNCO or a combinationthereof are effective for selectively reducing oxides of nitrogen whilethe exhaust gas is heated autothermally to a temperature in the range ofabout 1400-1550° F.

Yet other methods in accordance with the present invention include thefollowing. A method for autoignition of hydrocarbon(s) in exhaust gasesso that NH₃, HNCO or a combination thereof are effective for selectivelyreducing oxides of nitrogen while the exhaust gas is heatedautothermally to a temperature in the range of about 1400-1550° F.,substantially depleting residual concentrations of unburnedhydrocarbons, CO, HNCO and NH₃. A method of generating NH₃, HNCO or acombination thereof from chemical reductant(s) so that oxides ofnitrogen in an exhaust gas are reduced selectively while the exhaust gasis heated autothermally by hydrocarbon(s). A method of generating NH₃,HNCO or a combination thereof from chemical reductant(s) so that oxidesof nitrogen in an exhaust gas are reduced selectively while the exhaustgas is heated autothermally by hydrocarbon(s) to substantially depleteresidual concentrations of unburned hydrocarbons, CO, HNCO and NH₃. Amethod of generating NH₃, HNCO or a combination thereof from chemicalreductant(s) so that oxides of nitrogen in an exhaust gas are reducedselectively while the exhaust gas is heated autothermally byhydrocarbon(s) to a temperature in the range of about 1400-1550° F. Amethod of generating NH₃, HNCO or a combination thereof from chemicalreductant(s) so that oxides of nitrogen in an exhaust gas are reducedselectively while the exhaust gas is heated autothermally byhydrocarbon(s) to a temperature in the range of about 1400-1550° F.,substantially depleting residual concentrations of unburnedhydrocarbons, CO, HNCO and NH₃.

Still other methods in accordance with the present invention include thefollowing. A method for concurrently removing oxides of nitrogen andcarbon monoxide from an exhaust gas using NH₃, HNCO or a combinationthereof while hydrocarbon(s) autoignite and heat the exhaust gasautothermally. A method for concurrently removing oxides of nitrogen andcarbon monoxide from an exhaust gas using NH₃, HNCO or a combinationthereof while hydrocarbon(s) autoignite and heat the exhaust gasautothermally, substantially depleting residual concentrations ofunburned hydrocarbons, HNCO and NH₃. A method for concurrently removingoxides of nitrogen and carbon monoxide from an exhaust gas using NH₃,HNCO or a combination thereof while hydrocarbons(s) autoignite and heatthe exhaust gas autothermally to a temperature in the range of about1400-1550° F. A method for concurrently removing oxides of nitrogen andcarbon monoxide from an exhaust gas using NH₃, HNCO or a combinationthereof while hydrocarbons(s) autoignite and heat the exhaust gasautothermally to a temperature in the range of about 1400-1550° F.,substantially depleting residual concentrations of unburnedhydrocarbons, HNCO and NH₃. A method for treating an exhaust gas toremove oxides of nitrogen along with carbon monoxide and othercombustible contaminants using NH₃, HNCO or a combination thereof whilehydrocarbon(s) autoignite and heat the exhaust gas autothermally. Amethod for treating an exhaust gas to remove oxides of nitrogen alongwith carbon monoxide and other combustible contaminants using NH₃, HNCOor a combination thereof while hydrocarbon(s) autoignite and heat theexhaust gas autothermally, substantially depleting residualconcentrations of HNCO and NH₃. A method for treating an exhaust gas toremove oxides of nitrogen along with carbon monoxide and othercombustible contaminants using NH₃, HNCO or a combination thereof whilehydrocarbons(s) autoignite and heat the exhaust gas autothermally to atemperature in the range of about 1400-1550° F.

Other methods in accordance with the present invention include thefollowing. A method for treating an exhaust gas to remove oxides ofnitrogen along with carbon monoxide and other combustible contaminantsusing NH₃, HNCO or a combination thereof while hydrocarbons(s)autoignite and heat the exhaust gas autothermally to a temperature inthe range of about 1400-1550° F., substantially depleting residualconcentrations of HNCO and NH₃. A method for treating an exhaust gas toremove unburned hydrocarbons, CO and other combustible contaminantsusing hydrocarbon(s) to autoignite and heat the exhaust gasautothermally so that NH₃, HNCO or a combination thereof are alsoeffective for selectively reducing oxides of nitrogen in the exhaustgas. A method for treating an exhaust gas to remove unburnedhydrocarbons, CO and other combustible contaminants while concurrentlydepleting residual HNCO and NH₃ using hydrocarbon(s) to autoignite andheat the exhaust gas autothermally so that NH₃, HNCO or a combinationthereof are also effective for selectively reducing oxides of nitrogenin the exhaust gas. A method for treating an exhaust gas to removeunburned hydrocarbons, CO and other combustible contaminants usinghydrocarbon(s) to autoignite and heat the exhaust gas autothermally to atemperature in the range of about 1400-1550° F. so that NH₃, HNCO or acombination thereof are also effective for selectively reducing oxidesof nitrogen in the exhaust gas. A method for treating an exhaust gas toremove unburned hydrocarbons, CO and other combustible contaminantswhile concurrently depleting residual HNCO and NH₃ using hydrocarbon(s)to autoignite and heat the exhaust gas autothermally to a temperature inthe range of about 1400-1550° F. so that NH₃, HNCO or a combinationthereof are also effective for selectively reducing oxides of nitrogenin the exhaust gas. A method including the steps of: introducingreductant(s) for oxides of nitrogen into the exhaust gas so that thereductant(s) generate NH₃, HNCO or a combination thereof; introducinghydrocarbon(s) into an exhaust gas containing about 2-18% oxygen at atemperature in the range of about 900-1600° F. so that thehydrocarbon(s) autoignite in the exhaust gas; wherein the oxides ofnitrogen are reduced selectively while the exhaust gas is heatedautothermally by self-sustained autocatalytic reactions.

As also may be understood from the foregoing, in accordance withembodiments of the present invention: NOx in the exhaust gas may bereduced nearly stoichiometrically by as much as 80-90%; NOx in theexhaust gas may be selectively reduced by as much as about 99%; NOx inthe exhaust gas may be reduced to a level in the range of about 10-200ppm; and NH₃ and HNCO together may be in the range of about 0.5-2.0 or0.5-4.0 molar ratio with respect to the NOx in the exhaust gas. Highermolar ratios of NH₃ and HNCO together up to about 4.0 (e.g., from 0.5 to4.0, 0.75 to 4.0, from 0.75 to 3.5, about 3.0, about 3.5, about 4.0,etc.) with respect to NOx may generally increase NOx reduction withoutappreciably increasing NH₃ breakthrough using autocatalytic methods inaccordance with the present invention, but the cost-effectiveness ofsuch higher molar ratio embodiments diminishes when NH₃ and HNCO, etc.are used at levels in excess of twice the stoichiometric ratio withrespect to baseline NOx levels.

Additionally: the selective NOx reduction and the depletion of residualNH₃ and HNCO may contribute to the exhaust gas heating; the oxidation ofcombustible contaminants in the exhaust gas may contribute to exhaustgas heating; the exhaust gas temperature may be initially in the rangeof about 900-1600° F. and the exhaust gas initially may contain O₂ inthe range of about 2-18%; the autothermal heat release may be initiatedby autoignition of hydrocarbon(s) in the exhaust gas; the exhaust gasmay be heated substantially uniformly by the autoignition ofhydrocarbon(s) and the self-sustained autothermal heat release fromautocatalytic reactions; the autocatalytic reactions may beself-sustained in the autothermally heated exhaust gas even when aportion of the heat released is recovered by heat transfer surfaces; theheat release may be equivalent to a uniform adiabatic increase of about50-500° F. in the exhaust gas temperature; the exhaust gas may be heatedwithin about 0.02-1.5 seconds to a final temperature; the exhaust gasmay be heated to a final temperature in the range of about 1400-1550°F.; the exhaust gas may contain at least about 1% O₂ at a finaltemperature in the range of about 1400-1550° F.; residual concentrationsof hydrocarbons, CO, NH₃ and HNCO may be depleted substantially at afinal temperature in the range of about 1400-1550° F.; the CO may beoxidized below a residual concentration of about 2000 ppm; the CO may beoxidized below a residual concentration of about 500 ppm; the residualconcentration of CO may be maintained at a level below about 500 ppm bycontrolling the final exhaust gas temperature to be in the range ofabout 1400-1550° F.; and the residual concentration of NH₃ may bemaintained at a level below about 20 ppm by controlling the finalexhaust gas temperature in the range of about 1400-1550° F., or bycontrolling and/or monitoring the level of CO.

Additionally: the residual concentrations of CO and NH₃ may be depletedtogether autocatalytically in the exhaust gas at a final temperature inthe range of about 1400-1550° F.; the final temperature may becontrolled in the range of about 1400-1550° F. to maintain the residualCO concentration below about 50 ppm while depleting the residual NH₃below about 2 ppm; the hydrocarbon(s) may consist of material(s)selected from the group consisting of hydrocarbon mixtures such asnatural gas, liquefied petroleum gas, alcohols, gasoline, diesel fuel,aviation turbine fuel, various oxygenated hydrocarbons, hydrocarbonamines, or any fraction of such mixtures, including purified componentssuch as carbon monoxide, methane, propane, methanol, and ethanol, eitheras liquids or vapors; the hydrocarbons(s) may consist of the samematerial(s) as the liquid, gaseous, or vaporous fuel used to produce orotherwise heat the exhaust gas initially in the temperature range ofabout 900-1600° F.; the hydrocarbon(s) may be introduced into theexhaust gas substantially throughout a cross-section or around aperimeter of the exhaust gas flowpath using one or more nozzles,including a multiplicity thereof; the hydrocarbon(s) may be introducedas liquid drops with diameters in the range of about 20-500 microns; thehydrocarbon(s) may be introduced in the exhaust gas using a carrier oratomizing gas such as steam, compressed air, pressurized exhaust gas,gaseous or vaporous hydrocarbon(s) or any gaseous or vaporous NH₃compositions; the hydrocarbon(s) and any carrier or atomizing gas may bedistributed to an arrangement of one or more nozzles, including amultiplicity thereof; and the hydrocarbon(s) and any carrier oratomizing gas may be distributed in ways that prevent carbon formationor enhance the uniformity of autoignition and autothermal heating.

Additionally: the hydrocarbon(s) and any carrier or atomizing gas may bepressurized and/or metered; a final temperature may be measured at oneor more locations throughout the cross-section of the exhaust gasflowpath and the amount of introduced hydrocarbon(s) may be controlledto maintain the measured temperature(s) at a level in the range of about1400-1550° F.; the final CO concentration may be measured at one or morelocations throughout the cross-section of the exhaust gas flowpathdownstream from the temperature measurement(s); the final COmeasurement(s) may be used to verify a level of CO depletioncorresponding to the final temperature(s) for controlling theintroduction of hydrocarbon(s); NH₃, HNCO or a combination thereof maybe introduced into the exhaust gas; NH₃, HNCO or a combination thereofmay be generated by vaporization, decomposition or catalytic conversionof reductant(s) which may consist of material(s) selected from the groupconsisting of NH₃, HNCO, cyanuric acid or a tautomer of cyanuric acid,urea, decomposition products of urea, compounds which decompose toproduce NH₃ as a byproduct, ammonium salts of organic acids, hydrocarbonamines, or combinations of the foregoing, whether pure compounds ormixtures, as solids, liquid melts, emulsions, slurries, or solutions inwater, alcohols, hydrocarbons, or oxygenated hydrocarbon solvents; theNH₃, HNCO or a combination thereof may be generated prior to theintroduction of the hydrocarbon(s); and the NH₃, HNCO or a combinationthereof may be generated after the introduction of hydrocarbon(s); theNH₃, HNCO or a combination thereof may be generated concurrently withthe hydrocarbon(s).

Additionally: reductant(s) may be injected directly to vaporize ordecompose in the exhaust gas; the reductant(s) may be injected prior tothe introduction of hydrocarbon(s); the reductant(s) may be injectedafter the introduction of hydrocarbon(s); the reductant(s) may beinjected concurrently with the introduced hydrocarbon(s) as mixtures,solutions, emulsions, slurries, atomizing gases, atomized liquids orcombined chemical structures; the reductant(s) may be injectedsubstantially throughout a cross-section or around a perimeter of theexhaust gas flowpath using one or more nozzles, including a multiplicitythereof; the reductant(s) may be injected using a carrier or atomizinggas such as steam, compressed air, pressurized exhaust gas, gaseous orvaporous hydrocarbon(s) or any gaseous or vaporous NH₃ compositions; thereductant(s) are atomized to form liquid drops with diameters in therange of about 20-500 microns; the reductant(s) and any carrier oratomizing gas may be distributed to an arrangement of one or morenozzles, including a multiplicity thereof; the reductant(s) and anycarrier or atomizing gas may be distributed in ways that prevent theaccumulation of solid deposits and enhance the consistent introductionof NH₃, HNCO or a combination thereof in the exhaust gas; thereductant(s) may consist of a concentrated aqueous solution consistingof NH₃, urea or combinations thereof in water containing dissolvednitrogen in the range of about 15-30% by weight; a concentrated aqueoussolution of reductant(s) may be diluted with water; and thereductant(s), dilution water and any carrier or atomizing gas may bepressurized and/or metered.

Additionally: baseline NOx levels in the exhaust gas may be measuredwith respect to an operating condition of combustion equipment thatproduces the exhaust gas and the amount of reductant(s) injected tomaintain a predetermined level of NOx reduction or a predetermined finalNOx level in the exhaust gas may be controlled throughout the operatingrange of the combustion equipment; an operating condition of thecombustion equipment may be monitored to provide a basis for estimatingthe baseline NOx emissions throughout the operating range of thecombustion equipment, either continuously or on a periodic basis; theamount of injected reductant(s) may be controlled to generate NH₃ andHNCO together at a level in the range of about 0.5-2.0 molar ratio withrespect to the baseline NOx depending on a measurement of the operatingcondition for the combustion equipment; the final NOx level may bemeasured at one or more locations throughout the cross-section of theexhaust gas flowpath downstream from the autothermal heating and anytemperature measurement(s) may be used to control the introduction ofhydrocarbon(s); the final NOx level(s) may be used to verify theeffectiveness of selective NOx reduction corresponding to an amount ofintroduced NH₃ and HNCO or an amount of reductant(s) injected togenerate NH₃, HNCO or a combination thereof; the amount of introducedNH₃ and HNCO or the amount of reductant(s) injected may be controlled tomaintain a predetermined final NOx level in the exhaust gas; and theamount of introduced NH₃ and HNCO together or the amount of reductant(s)injected may be increased or decreased depending on the need to lower orraise, respectively, the measured final NOx level with respect to adesired final level of NOx in the exhaust gas.

Additionally: the dilution water and any carrier or atomizing gas may betreated to prevent the formation of deposits or scale in surge tanks,piping, and other equipment or instrumentation used to pressurize,meter, transport, distribute, and inject the dilution water, carriergas, or atomizing gas; concentrated aqueous solutions consisting of NH₃,urea, or combinations thereof may be formulated and/or prepared in watercontaining dissolved nitrogen in the range of about 15-30% by weight;the concentrated aqueous solution may be prepared using distilled orde-ionized water to prevent the formation of deposits or scale instorage tanks, piping and other equipment or instrumentation used topressurize, meter, transport, distribute, and inject the dissolvedreductant(s) as concentrated or diluted solutions in water; theconcentrated aqueous solution may contain chemical additives to preventthe formation of deposits or scale in storage tanks, piping, and otherequipment or instrumentation used to pressurize, meter, transport,distribute and inject the dissolved reductant(s) as concentrated ordiluted solutions in water.

Additionally: the exhaust gas may be preheated or precooled to atemperature in the range of about 900-1600° F. before the introductionof hydrocarbon(s); the exhaust gas may be preheated or precooled to atemperature in the range of about 1050-1600° F. so that the exhaust gasis heated autothermally in about 0.02-1.0 seconds to a final temperaturein the range of about 1400-1550° F. by a heat release equivalent to anadiabatic increase of about 50-350° F. effective for enhancing theselectivity of autocatalytic NOx reduction; the exhaust gas may bepreheated or precooled to a temperature in the range of about 1200-1600°F. so that the exhaust gas is heated autothermally in about 0.02-0.5seconds to a final temperature in the range of about 1400-1550° F. by aheat release equivalent to an adiabatic increase of about 50-200° F.effective for enhancing the selectivity of autocatalytic NOx reduction;the exhaust gas temperature may be controlled at a level in the range ofabout 900-1600° F. before the introduction of hydrocarbon(s); and heatmay be recovered from the autothermally heated exhaust gas using heattransfer surfaces to preheat the exhaust gas in the temperature range ofabout 900-1350° F.

Additionally: the temperature of the autothermally heated exhaust gasmay be controlled at a level in the range of about 1400-1550° F. inorder to maintain the preheated exhaust gas temperature in the range ofabout 900-1350° F. using a heat exchanger; a supplemental fuel may becombusted to preheat the exhaust gas in the temperature range of about900-1350° F.; a supplemental fuel may be combusted directly in theexhaust gas; the exhaust gas may be preheated by mixing with theproducts from combustion of supplemental fuel and air; a supplementalfuel may be combusted directly in the exhaust gas after a portion of theexhaust gas is preheated by mixing with the products from combustion ofsupplemental fuel and air; the supplemental fuel may consist of the samehydrocarbon(s) which autoignite and maintain the autocatalytic heatrelease in the exhaust gas; the supplemental fuel may consist of thesame material(s) used to produce the exhaust gas; and the exhaust gasmay be enriched to contain in the range of about 2-5% O₂ using excessair for the combustion of supplemental fuel.

Additionally: the exhaust gas flowpath may be modified to mix theinjected materials(s) as well as the exhaust gas composition; theexhaust gas flowpath may be modified before the introduction ofhydrocarbon(s); the exhaust gas flowpath may be modified after theintroduction of hydrocarbon(s); the exhaust gas flowpath may be modifiedboth before and after the introduction of hydrocarbon(s); the mixingeffects may extend substantially throughout the cross-section of theexhaust gas flowpath and may continue to mix the exhaust gas compositionfor a portion, or even all, of the autothermal heating; vanes may beused to swirl the gas flowpath, either as a single swirl flow or as amultiplicity of mixing swirl flows; baffles may be used to generate amultiplicity of turbulent mixing eddies; and the introducedhydrocarbon(s) may be mixed substantially uniformly before theappearance of a visible chemiluminescence.

Additionally: heat may be recovered from the autothermally heatedexhaust gas using heat transfer surfaces between the exhaust gas and theheat recovery fluid; heat may be recovered from the exhaust gas to heatanother fluid such as steam, water, combustion air or a petrochemicalcomposition; a petrochemical composition may be cracked; steam may begenerated; and the steam may be coupled to a turbine and operatemachinery or generate electricity.

Additionally: the production of the exhaust gas containing NOx may bemodified to maintain a temperature in the range of about 900-1600° F.and to contain in the range of about 2-18% O₂; the production of theexhaust gas may be modified by staging the primary combustion usingsecondary air or over-fire air to decrease the formation of NOx; thefuel consumed to produce the exhaust gas may be reduced by an amount ofheat release less than or equal to the heat released by autocatalyticreactions in the exhaust gas; O₂ enrichment of the primary combustionmay decrease the formation of combustible contaminants in the exhaustgas, including gaseous organics, soot, and unburned carbonaceousmaterials or particulate matter; and the O₂ enrichment of coalcombustion may decrease unburned carbon on fly ash.

Additionally: a fuel may be combusted in a heat exchange boiler and theexhaust gas may be cooled before the introduction of hydrocarbon(s)which autoignite and maintain autocatalytic heat release in the exhaustgas; a petrochemical composition may be cracked; steam may be generated;the steam may be coupled to a turbine and operate machinery or generateelectricity; a fuel may be combusted in an internal combustion engine;the internal combustion engine may be coupled to an electric generator,and electricity may be generated; the internal combustion engine may becoupled to a pump or compressor, and pressure and/or fluid flow may begenerated using the pump or compressor; the internal combustion enginemay be coupled to machinery, and the machinery may be operated so as toproduce mineral resources; the internal combustion engine may be coupledto power mobile equipment, and personnel, materials, products orminerals may be transported and/or processed; and waste may be combustedin a waste to heat incinerator, and combustible waste may be disposedof.

Additionally: NH₃ breakthrough may be generated from a previousnoncatalytic process for selectively reducing NOx, and NOx may benoncatalytically reduced at temperatures above 1600° F. prior to theautothermal exhaust gas heating and autocatalytic NOx reduction; aportion of the remaining NOx may be catalytically reduced using a solidcatalytic surface and additional generation of NH₃, HNCO or combinationsthereof after the autothermal exhaust gas heating; the exhaust gas maybe heated autothermally to control the temperature for the subsequentcatalytic NOx reduction using a solid catalytic surface; the first stageof gas-phase autocatalytic NOx reduction may lower the inlet NOx levelto the solid catalytic surface; the first stage of autothermal heatingmay decrease exhaust gas contaminants such as hydrocarbons, soot, CO andparticulate matter; and the solid catalytic surface may serve the dualpurpose of decreasing the final NOx level and recovering a portion ofthe autothermal heat release.

Further, reductant(s) may be injected or generated incrementally as theexhaust gas is autothemally heated. Staging the injection or generationof reductant(s) replenishes NH₃, HNCO, or combinations thereof in theexhaust gas as reactants are consumed for NOx reduction. This staging iseffective for NOx reduction so long as the autocatalytic reactionscontinue to heat the exhaust gas. The injection of reductant(s) may bestaged using one or more nozzles, including a multiplicity thereof,along the exhaust gas flowpath. Alternatively, NH₃, HNCO, or acombination thereof may be generated incrementally by the decompositionor vaporization of reductant(s) which are injected as solids, liquidmelts, emulsions, slurries, or solutions. This alternative staging mayinvolve different reductant(s), solution concentrations, and spraydroplet sizes in order to modify the generation of NH₃, HNCO, or acombination thereof during the autothermal heating of the exhaust gas.For this staging, the reductant(s) may consist of a dilute aqueoussolution consisting of NH₃, urea, or a combination thereof in watercontaining dissolved nitrogen in the range of about 2-15% by weight.

From the foregoing, it will be understood and appreciated by thoseskilled in the art that the embodiments of the present invention may beutilized in a wide variety of applications and in modifiedconfigurations and the like.

Although various preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and/or substitutionsare possible without departing from the scope and spirit of the presentinvention as disclosed in the claims.

What is claimed is:
 1. A method for reducing NOx in an exhaust gas,comprising contacting the exhaust gas with a reductant effective for NOxreduction under autocatalytic conditions at a location in the exhaustgas flowpath where the exhaust gas is autothermally heated to achievesaid autocatalytic conditions using one or more hydrocarbons, whereinthe reductant comprises ammonia.
 2. The method of claim 1, wherein apressurized gas is used to convey or atomize the reductant and/or thehydrocarbon(s) into the exhaust gas flowpath.
 3. The method of claim 2,wherein the pressurized gas is selected from the group consisting ofsteam, compressed air, pressurized exhaust gas, a gaseous or vaporousform of the hydrocarbon(s) and a gaseous or vaporous form of thereductant.
 4. The method of claim 3, wherein the pressurized gas is agaseous or vaporous form of the hydrocarbon(s).
 5. The method of claim3, wherein the pressurized gas is a gaseous or vaporous form of thereductant.
 6. The method of claim 1, wherein the content of NOx in theexhaust gas is reduced by 80-90% in nearly stoichiometric proportion tothe amount of added reductant.
 7. The method of claim 1, wherein the NOxin the exhaust gas is selectively reduced by as much as about 99%. 8.The method of claim 1, wherein the NOx in the exhaust gas is reduced toa level in the range of about 10-200 ppm.
 9. The method of claim 1,wherein the autothermal heat release is initiated by autoignition ofhydrocarbon(s) in the exhaust gas.
 10. The method of claim 1, whereinthe heat release is equivalent to a uniform adiabatic increase of about50-500° F. in the exhaust gas temperature.
 11. The method of claim 1,wherein the CO is oxidized below a residual concentration of about 500ppm.
 12. The method of claim 1, wherein the hydrocarbon(s) comprise amember selected from the group consisting of natural gas, liquefiedpetroleum gas, alcohols, gasoline, diesel fuel, aviation turbine fuel,oxygenated hydrocarbons, hydrocarbon amines, methane, propane, methanoland ethanol.
 13. The method of claim 1, wherein the hydrocarbon(s) areintroduced throughout a cross-section or around a perimeter of theexhaust gas flowpath using one or more nozzles.
 14. The method of claim1, wherein the reductant is introduced throughout a cross-section oraround a perimeter of the exhaust gas flowpath using one or morenozzles.
 15. The method of claim 1, further comprising the steps ofmeasuring a final temperature at one or more locations throughout thecross-section of the exhaust gas flowpath and controlling the amount ofhydrocarbon(s) in the exhaust gas to maintain the measuredtemperature(s) at a level in the range of about 1400-1550° F.
 16. Amethod for reducing NOx in an exhaust gas, comprising contacting theexhaust gas with a reductant effective for NOx reduction underautocatalytic conditions at a location in the exhaust gas flowpath wherethe exhaust gas is autothermally heated to achieve said autocatalyticconditions using one or more hydrocarbons, and wherein the reductantcomprises urea.
 17. The method of claim 16, wherein a pressurized gas isused to convey or atomize the reductant and/or the hydrocarbon(s) intothe exhaust gas flowpath.
 18. The method of claim 17, wherein thepressurized gas is selected from the group consisting of steam,compressed air, pressurized exhaust gas, a gaseous or vaporous form ofthe hydrocarbon(s) and a gaseous or vaporous form of the reductant. 19.The method of claim 18, wherein the pressurized gas is a gaseous orvaporous form of the hydrocarbon(s).
 20. The method of claim 18, whereinthe pressurized gas is a gaseous or vaporous form of the reductant. 21.The method of claim 16, wherein the content of NOx in the exhaust gas isreduced by 80-90% in nearly stoichiometric proportion to the amount ofadded reductant.
 22. The method of claim 16, wherein the NOx in theexhaust gas is selectively reduced by as much as about 99%.
 23. Themethod of claim 16, wherein the NOx in the exhaust gas is reduced to alevel in the range of about 10-200 ppm.
 24. The method of claim 16,wherein the autothermal heat release is initiated by autoignition ofhydrocarbon(s) in the exhaust gas.
 25. The method of claim 16, whereinthe heat release is equivalent to a uniform adiabatic increase of about50-500° F. in the exhaust gas temperature.
 26. The method of claim 16,wherein the CO is oxidized below a residual concentration of about 500ppm.
 27. The method of claim 16, wherein the hydrocarbon(s) comprise amember selected from the group consisting of natural gas, liquefiedpetroleum gas, alcohols, gasoline, diesel fuel, aviation turbine fuel,oxygenated hydrocarbons, hydrocarbon amines, methane, propane, methanoland ethanol.
 28. The method of claim 16, wherein the hydrocarbon(s) areintroduced throughout a cross-section or around a perimeter of theexhaust gas flowpath using one or more nozzles.
 29. The method of claim16, wherein the reductant is introduced throughout a cross-section oraround a perimeter of the exhaust gas flowpath using one or morenozzles.
 30. The method of claim 16, further comprising the steps ofmeasuring a final temperature at one or more locations throughout thecross-section of the exhaust gas flowpath and controlling the amount ofhydrocarbon(s) in the exhaust gas to maintain the measuredtemperature(s).at a level in the range of about 1400-1550° F.
 31. Amethod for reducing NOx in an exhaust gas, comprising contacting theexhaust gas with a reductant effective for NOx reduction underautocatalytic conditions at a location in the exhaust gas flowpath wherethe exhaust gas is autothermally heated to achieve said autocatalyticconditions using one or more hydrocarbons, wherein the reductantcomprises HNCO.
 32. The method of claim 31, wherein a pressurized gas isused to convey or atomize the reductant and/or the hydrocarbon(s) intothe exhaust gas flowpath.
 33. The method of claim 32, wherein thepressurized gas is selected from the group consisting of steam,compressed air, pressurized exhaust gas, a gaseous or vaporous form ofthe hydrocarbon(s) and a gaseous or vaporous form of the reductant. 34.The method of claim 33, wherein the pressurized gas is a gaseous orvaporous form of the hydrocarbon(s).
 35. The method of claim 33, whereinthe-pressurized gas is a gaseous or vaporous form of the reductant. 36.The method of claim 31, wherein the content of NOx in the exhaust gas isreduced by 80-90% in nearly stoichiometric proportion to the amount ofadded reductant.
 37. The method of claim 31 wherein the NOx in theexhaust gas is selectively reduced by as much as about 99%.
 38. Themethod of claim 31, wherein the NOx in the exhaust gas is reduced to alevel in the range of about 10-200 ppm.
 39. The method of claim 31,wherein the autothermal heat release is initiated by autoignition ofhydrocarbon(s) in the exhaust gas.
 40. The method of claim 31, whereinthe heat release is equivalent to a uniform adiabatic increase of about50-500° F. in the exhaust gas temperature.
 41. The method of claim 31,wherein the CO is oxidized below a residual concentration of about 500ppm.
 42. The method of claim 31, wherein the hydrocarbon(s) comprise amember selected from the group consisting of natural gas, liquefiedpetroleum gas, alcohols, gasoline, diesel fuel, aviation turbine fuel,oxygenated hydrocarbons, hydrocarbon amines, methane, propane, methanoland ethanol.
 43. The method of claim 31, wherein the hydrocarbon(s) areintroduced throughout a cross-section or around a perimeter of theexhaust gas flowpath using one or more nozzles.
 44. The method of claim31, wherein the reductant is introduced throughout a cross-section oraround a perimeter of the exhaust gas flowpath using one or morenozzles.
 45. The method of claim 31, further comprising the steps ofmeasuring a final temperature at one or more locations throughout thecross-section of the exhaust gas flowpath and controlling the amount ofhydrocarbon(s) in the exhaust gas to maintain the measuredtemperature(s) at a level in the range of about 1400-1550° F.
 46. Amethod for determining if a pollutant in an exhaust gas stream has beendepleted by an autocatalytic process, comprising: introducing one ormore hydrocarbon(s) into the exhaust gas stream at a first location toautothermally heat the exhaust gas in the presence of a reductanteffective for reduction of NOx in the exhaust gas; measuring CO at asecond location in the exhaust gas stream downstream from the firstlocation; determining if the level of CO at the second location exceedsa predetermined level that corresponds to a final temperature of theexhaust gas sufficient for autocatalytic depletion of the pollutant. 47.The method of claim 46, wherein the level of CO measured at the secondlocation is used to determine if NH₃ breakthrough has occurred.
 48. Themethod of claim 46, further comprising increasing the amount ofhydrocarbon(s) introduced at the first location if the level of CO atthe second location exceeds the predetermined level.
 49. The method ofclaim 46, wherein the pollutant is CO.
 50. The method of claim 46,wherein t he pollutant is NH₃.
 51. The method of claim 46, wherein thepollutant is NOx.
 52. A method for determining a level of NOx depletionby an autocatalytic process in an exhaust gas stream, comprising:contacting the exhaust gas stream with one or more hydrocarbon(s) andone or more reductant(s) effective for reduction of NOx at a firstlocation so that the exhaust gas is autothermally heated toautocatalytically deplete NOx in the gas stream; measuring the amount ofNOx at a second location in the exhaust gas stream downstream from thefirst location.
 53. The method of claim 52, further comprisingincreasing the amount of said reductant(s) at the first location tolower the amount of NOx at the second location.
 54. The method of claim52, further comprising decreasing the amount of said reductant(s) at thefirst location to raise the amount of NOx at the second location. 55.The method of claim 52, wherein the NOx is measured continuously. 56.The method of claim 52, wherein the NOx is measured periodically.